Align core lease and amendment validation (#49)

2026-08-17 14:18:16 +02:00
1 parent 96bd02e3a6
commit ca3e28d3a2
@@ -0,0 +1,165 @@
+<section class="form-page">
+	<header class="page-heading">
+		<div>
+			<button
+				mat-button
+				type="button"
+				class="back-action"
+				[disabled]="saving()"
+				(click)="cancelled.emit()"
+			>
+				← Retour
+			</button>
+			<p class="eyebrow">Bail actif</p>
+			<h1>Préparer un avenant</h1>
+			<p>
+				Seuls les locataires, garants, terme et conditions de paiement peuvent être
+				modifiés.
+			</p>
+		</div>
+	</header>
+	<form
+		class="entity-form"
+		[formRoot]="amendmentForm"
+		(ngSubmit)="save()"
+		[attr.aria-busy]="saving()"
+	>
+		@if (showSaveError()) {
+			<div class="alert-error" role="alert">{{ saveError() }}</div>
+		}
+		<fieldset class="busy-fieldset" [disabled]="saving()" [attr.inert]="saving() ? '' : null">
+			<div class="form-grid">
+				<mat-form-field appearance="outline">
+					<mat-label>Prise d’effet</mat-label>
+					<input
+						matInput
+						id="effectiveDate"
+						type="date"
+						[formField]="amendmentForm.effectiveDate"
+						(change)="fieldWasChanged('effectiveDate')"
+					/>
+					@if (fieldError('effectiveDate')) {
+						<mat-error>{{ fieldError('effectiveDate') }}</mat-error>
+					}
+				</mat-form-field>
+				<mat-form-field appearance="outline">
+					<mat-label>Locataires</mat-label>
+					<mat-select
+						id="tenantIds"
+						[formField]="amendmentForm.tenantIds"
+						multiple
+						(selectionChange)="fieldWasChanged('tenantIds')"
+					>
+						@for (tenant of tenants(); track tenant.id) {
+							<mat-option [value]="tenant.id"
+								>{{ tenant.firstName }} {{ tenant.lastName }}</mat-option
+							>
+						}
+					</mat-select>
+					@if (fieldError('tenantIds')) {
+						<mat-error>{{ fieldError('tenantIds') }}</mat-error>
+					}
+				</mat-form-field>
+				<mat-form-field appearance="outline">
+					<mat-label>Fin de bail</mat-label>
+					<input
+						matInput
+						id="endDate"
+						type="date"
+						[formField]="amendmentForm.endDate"
+						(change)="fieldWasChanged('endDate')"
+					/>
+					@if (fieldError('endDate')) {
+						<mat-error>{{ fieldError('endDate') }}</mat-error>
+					}
+				</mat-form-field>
+				<mat-form-field appearance="outline">
+					<mat-label>Loyer mensuel</mat-label>
+					<input
+						matInput
+						id="monthlyRent"
+						type="number"
+						step="0.01"
+						[formField]="amendmentForm.monthlyRent"
+						(change)="fieldWasChanged('monthlyRent')"
+					/>
+					<span matTextSuffix>€</span>
+					@if (fieldError('monthlyRent')) {
+						<mat-error>{{ fieldError('monthlyRent') }}</mat-error>
+					}
+				</mat-form-field>
+				<mat-form-field appearance="outline">
+					<mat-label>Charges mensuelles</mat-label>
+					<input
+						matInput
+						id="monthlyCharges"
+						type="number"
+						step="0.01"
+						[formField]="amendmentForm.monthlyCharges"
+						(change)="fieldWasChanged('monthlyCharges')"
+					/>
+					<span matTextSuffix>€</span>
+					@if (fieldError('monthlyCharges')) {
+						<mat-error>{{ fieldError('monthlyCharges') }}</mat-error>
+					}
+				</mat-form-field>
+				<mat-form-field appearance="outline">
+					<mat-label>Mode de récupération des charges</mat-label>
+					<mat-select
+						id="chargeMethod"
+						[formField]="amendmentForm.chargeMethod"
+						(selectionChange)="fieldWasChanged('chargeMethod')"
+					>
+						<mat-option value="FLAT_RATE">Forfait de charges</mat-option>
+						<mat-option value="PROVISION"
+							>Provision avec régularisation annuelle</mat-option
+						>
+					</mat-select>
+				</mat-form-field>
+				<mat-form-field appearance="outline">
+					<mat-label>Jour d’échéance</mat-label>
+					<input
+						matInput
+						id="dueDay"
+						type="number"
+						step="1"
+						[formField]="amendmentForm.dueDay"
+						(change)="fieldWasChanged('dueDay')"
+					/>
+					@if (fieldError('dueDay')) {
+						<mat-error>{{ fieldError('dueDay') }}</mat-error>
+					}
+				</mat-form-field>
+				@if (amendmentModel().chargeMethod === 'FLAT_RATE') {
+					<mat-checkbox id="indexFlatRate" [formField]="amendmentForm.indexFlatRate"
+						>Indexer le forfait de charges avec le loyer</mat-checkbox
+					>
+				}
+			</div>
+			<section class="guarantor-section">
+				<h2>Garants</h2>
+				@for (guarantor of guarantors(); track $index) {
+					<div class="guarantor-chip">
+						<span>{{ guarantor.firstName }} {{ guarantor.lastName }}</span>
+						<button mat-button type="button" (click)="removeGuarantor($index)">
+							Retirer
+						</button>
+					</div>
+				}
+				<app-guarantor-form (submitted)="addGuarantor($event)" />
+			</section>
+			<div class="form-actions">
+				<button mat-button type="button" [disabled]="saving()" (click)="cancelled.emit()">
+					Annuler
+				</button>
+				<button mat-flat-button type="submit" class="primary-action" [disabled]="saving()">
+					@if (saving()) {
+						<mat-spinner diameter="18" aria-hidden="true" /> Enregistrement…
+					} @else {
+						Enregistrer l’avenant
+					}
+				</button>
+			</div>
+		</fieldset>
+	</form>
+</section>
@@ -0,0 +1,182 @@
+import { TestBed } from '@angular/core/testing';
+import { vi } from 'vitest';
+import { emptyLeaseLegalTerms } from '../../services/lease.service';
+import { LeaseAmendmentForm } from './lease-amendment-form';
+
+describe('LeaseAmendmentForm', () => {
+	afterEach(() => vi.useRealTimers());
+
+	it('submits only mutable contract facts', () => {
+		const fixture = TestBed.createComponent(LeaseAmendmentForm);
+		fixture.componentRef.setInput('tenants', [
+			{
+				id: 'tenant-1',
+				firstName: 'Camille',
+				lastName: 'Martin',
+				email: '',
+				phone: '',
+				address: '',
+			},
+		]);
+		fixture.componentRef.setInput('lease', {
+			id: 'lease-1',
+			apartmentId: 'apartment-1',
+			tenantIds: ['tenant-1'],
+			guarantors: [],
+			specialClauses: [],
+			legalTerms: emptyLeaseLegalTerms(),
+			leaseType: 'STANDARD',
+			startDate: '2026-09-01',
+			endDate: '2027-08-31',
+			monthlyRent: 850,
+			monthlyCharges: 50,
+			securityDeposit: 850,
+			dueDay: 5,
+			chargeMethod: 'FLAT_RATE',
+			indexFlatRate: false,
+			status: 'ACTIVE',
+		});
+		fixture.componentInstance.amendmentModel.update((model) => ({
+			...model,
+			effectiveDate: '2026-10-01',
+			monthlyRent: 0.01,
+			monthlyCharges: 0,
+			dueDay: 28,
+		}));
+		const submitted = vi.fn();
+		fixture.componentInstance.submitted.subscribe(submitted);
+		fixture.detectChanges();
+
+		fixture.componentInstance.save();
+
+		expect(submitted).toHaveBeenCalledWith(
+			expect.objectContaining({
+				effectiveDate: '2026-10-01',
+				tenantIds: ['tenant-1'],
+				endDate: '2027-08-31',
+				monthlyRent: 0.01,
+				monthlyCharges: 0,
+				dueDay: 28,
+			}),
+		);
+		expect(submitted.mock.calls[0][0]).not.toHaveProperty('securityDeposit');
+		expect(submitted.mock.calls[0][0]).not.toHaveProperty('apartmentId');
+	});
+
+	it('blocks a date before the amendment takes effect', () => {
+		const fixture = TestBed.createComponent(LeaseAmendmentForm);
+		fixture.componentRef.setInput('tenants', []);
+		fixture.componentRef.setInput('lease', {
+			id: 'lease-1',
+			apartmentId: 'apartment-1',
+			tenantIds: ['tenant-1'],
+			guarantors: [],
+			specialClauses: [],
+			legalTerms: emptyLeaseLegalTerms(),
+			leaseType: 'STANDARD',
+			startDate: '2026-09-01',
+			endDate: '2027-08-31',
+			monthlyRent: 850,
+			monthlyCharges: 50,
+			securityDeposit: 850,
+			dueDay: 5,
+			chargeMethod: 'FLAT_RATE',
+			indexFlatRate: false,
+			status: 'ACTIVE',
+		});
+		fixture.componentInstance.amendmentModel.update((model) => ({
+			...model,
+			effectiveDate: '2026-10-01',
+			endDate: '2026-09-30',
+		}));
+		const submitted = vi.fn();
+		fixture.componentInstance.submitted.subscribe(submitted);
+		fixture.detectChanges();
+
+		fixture.componentInstance.save();
+
+		expect(submitted).not.toHaveBeenCalled();
+		expect(fixture.componentInstance.fieldError('endDate')).toContain('ne peut pas précéder');
+	});
+
+	it('blocks a past or out-of-period effective date and focuses it', async () => {
+		vi.useFakeTimers();
+		vi.setSystemTime(new Date('2026-07-24T12:00:00'));
+		const fixture = TestBed.createComponent(LeaseAmendmentForm);
+		fixture.componentRef.setInput('tenants', []);
+		fixture.componentRef.setInput('lease', {
+			id: 'lease-1',
+			apartmentId: 'apartment-1',
+			tenantIds: ['tenant-1'],
+			guarantors: [],
+			specialClauses: [],
+			legalTerms: emptyLeaseLegalTerms(),
+			leaseType: 'STANDARD',
+			startDate: '2026-07-01',
+			endDate: '2027-06-30',
+			monthlyRent: 850,
+			monthlyCharges: 50,
+			securityDeposit: 850,
+			dueDay: 5,
+			chargeMethod: 'FLAT_RATE',
+			indexFlatRate: false,
+			status: 'ACTIVE',
+		});
+		fixture.componentInstance.amendmentModel.update((model) => ({
+			...model,
+			effectiveDate: '2026-07-23',
+			endDate: '2027-06-30',
+		}));
+		fixture.detectChanges();
+
+		fixture.componentInstance.save();
+		await new Promise<void>((resolve) => queueMicrotask(resolve));
+		expect(fixture.componentInstance.fieldError('effectiveDate')).toContain(
+			'entre aujourd’hui',
+		);
+		expect(document.activeElement?.id).toBe('effectiveDate');
+
+		fixture.componentInstance.amendmentModel.update((model) => ({
+			...model,
+			effectiveDate: '2027-07-01',
+		}));
+		expect(fixture.componentInstance.fieldError('effectiveDate')).toContain('fin du bail');
+	});
+
+	it('clears virtual tenant and end-date error paths with their visible controls', () => {
+		const fixture = TestBed.createComponent(LeaseAmendmentForm);
+		fixture.componentRef.setInput('tenants', []);
+		fixture.componentRef.setInput('lease', {
+			id: 'lease-1',
+			apartmentId: 'apartment-1',
+			tenantIds: ['tenant-1'],
+			guarantors: [],
+			specialClauses: [],
+			legalTerms: emptyLeaseLegalTerms(),
+			leaseType: 'STANDARD',
+			startDate: '2026-09-01',
+			endDate: '2027-08-31',
+			monthlyRent: 850,
+			monthlyCharges: 50,
+			securityDeposit: 850,
+			dueDay: 5,
+			chargeMethod: 'FLAT_RATE',
+			indexFlatRate: false,
+			status: 'ACTIVE',
+		});
+		fixture.componentRef.setInput('saveFieldErrors', [
+			{ field: 'tenantIdsDistinct', code: 'assert_true' },
+			{ field: 'endDateOnOrAfterEffectiveDate', code: 'assert_true' },
+		]);
+		const changed = vi.fn();
+		fixture.componentInstance.fieldChanged.subscribe(changed);
+		fixture.detectChanges();
+
+		expect(fixture.componentInstance.fieldError('tenantIds')).toContain('sans doublon');
+		expect(fixture.componentInstance.fieldError('endDate')).toContain('ne peut pas précéder');
+		fixture.componentInstance.fieldWasChanged('tenantIds');
+		fixture.componentInstance.fieldWasChanged('endDate');
+		expect(changed).toHaveBeenCalledWith('tenantIdsDistinct');
+		expect(changed).toHaveBeenCalledWith('endDateOnOrAfterEffectiveDate');
+	});
+});
@@ -0,0 +1,265 @@
+import { Component, effect, ElementRef, inject, input, output, signal } from '@angular/core';
+import { FormsModule } from '@angular/forms';
+import { FormField, FormRoot, form, max, min, required, validate } from '@angular/forms/signals';
+import { MatButtonModule } from '@angular/material/button';
+import { MatCheckboxModule } from '@angular/material/checkbox';
+import { MatFormFieldModule } from '@angular/material/form-field';
+import { MatInputModule } from '@angular/material/input';
+import { MatProgressSpinnerModule } from '@angular/material/progress-spinner';
+import { MatSelectModule } from '@angular/material/select';
+import { ChargeMethod, Guarantor, Lease, LeaseAmendmentInput } from '../../services/lease.service';
+import { FieldValidationError } from '../../services/http-error-message';
+import { Tenant } from '../../services/tenant.service';
+import { GuarantorForm } from '../guarantor-form/guarantor-form';
+
+type AmendmentField =
+	'effectiveDate' | 'tenantIds' | 'endDate' | 'monthlyRent' | 'monthlyCharges' | 'dueDay';
+
+const serverErrorCodes: Record<AmendmentField, ReadonlySet<string>> = {
+	effectiveDate: new Set(['not_null', 'not_past']),
+	tenantIds: new Set(['not_empty', 'assert_true']),
+	endDate: new Set(['not_null', 'assert_true']),
+	monthlyRent: new Set(['positive']),
+	monthlyCharges: new Set(['positive_or_zero']),
+	dueDay: new Set(['min', 'max']),
+};
+
+const amendmentFields: AmendmentField[] = [
+	'effectiveDate',
+	'tenantIds',
+	'endDate',
+	'monthlyRent',
+	'monthlyCharges',
+	'dueDay',
+];
+
+@Component({
+	selector: 'app-lease-amendment-form',
+	imports: [
+		FormsModule,
+		FormRoot,
+		FormField,
+		MatButtonModule,
+		MatCheckboxModule,
+		MatFormFieldModule,
+		MatInputModule,
+		MatProgressSpinnerModule,
+		MatSelectModule,
+		GuarantorForm,
+	],
+	templateUrl: './lease-amendment-form.html',
+})
+export class LeaseAmendmentForm {
+	private readonly element = inject<ElementRef<HTMLElement>>(ElementRef);
+	readonly lease = input.required<Lease>();
+	readonly tenants = input.required<Tenant[]>();
+	readonly saving = input(false);
+	readonly saveError = input('');
+	readonly saveFieldErrors = input<FieldValidationError[]>([]);
+	readonly cancelled = output<void>();
+	readonly submitted = output<LeaseAmendmentInput>();
+	readonly fieldChanged = output<string>();
+	private previousServerErrorSignature = '';
+
+	readonly amendmentModel = signal({
+		effectiveDate: '',
+		tenantIds: [] as string[],
+		endDate: '',
+		monthlyRent: 0,
+		monthlyCharges: 0,
+		dueDay: 5,
+		chargeMethod: 'FLAT_RATE' as ChargeMethod,
+		indexFlatRate: false,
+	});
+	readonly guarantors = signal<Guarantor[]>([]);
+	readonly amendmentForm = form(this.amendmentModel, (f) => {
+		required(f.effectiveDate);
+		validate(f.effectiveDate, ({ value, valueOf }) => {
+			const effectiveDate = value();
+			const endDate = valueOf(f.endDate);
+			return !effectiveDate ||
+				(effectiveDate >= browserToday() &&
+					effectiveDate >= this.lease().startDate &&
+					effectiveDate <= this.lease().endDate &&
+					(!endDate || effectiveDate <= endDate))
+				? null
+				: {
+						kind: 'amendment-effective-date',
+						message:
+							'La prise d’effet doit être comprise entre aujourd’hui et la fin du bail, sans dépasser sa nouvelle fin.',
+					};
+		});
+		required(f.tenantIds);
+		required(f.endDate);
+		validate(f.endDate, ({ value, valueOf }) => {
+			const endDate = value();
+			const effectiveDate = valueOf(f.effectiveDate);
+			return !endDate ||
+				(endDate >= this.lease().startDate && (!effectiveDate || endDate >= effectiveDate))
+				? null
+				: {
+						kind: 'amendment-end-date',
+						message:
+							'La fin du bail ne peut pas précéder sa prise d’effet ni son début.',
+					};
+		});
+		required(f.monthlyRent);
+		min(f.monthlyRent, Number.MIN_VALUE);
+		required(f.monthlyCharges);
+		min(f.monthlyCharges, 0);
+		required(f.dueDay);
+		min(f.dueDay, 1);
+		max(f.dueDay, 28);
+		validate(f.dueDay, ({ value }) =>
+			Number.isInteger(value())
+				? null
+				: { kind: 'whole-day', message: 'Le jour d’échéance doit être un nombre entier.' },
+		);
+	});
+
+	constructor() {
+		effect(() => this.reset(this.lease()));
+		effect(() => {
+			const errors = this.saveFieldErrors();
+			const signature = errors
+				.map((error) => `${error.field}:${error.code}`)
+				.sort()
+				.join('|');
+			if (signature && signature !== this.previousServerErrorSignature) {
+				queueMicrotask(() => this.focusFirstInvalid());
+			}
+			this.previousServerErrorSignature = signature;
+		});
+	}
+
+	save(): void {
+		if (this.saving()) return;
+		this.amendmentForm().markAsTouched();
+		if (this.amendmentForm().invalid()) {
+			queueMicrotask(() => this.focusFirstInvalid());
+			return;
+		}
+		const model = this.amendmentModel();
+		this.submitted.emit({
+			...model,
+			timeZone: Intl.DateTimeFormat().resolvedOptions().timeZone || 'Europe/Paris',
+			guarantors: this.guarantors(),
+			indexFlatRate: model.chargeMethod === 'FLAT_RATE' && model.indexFlatRate,
+		});
+	}
+
+	addGuarantor(guarantor: Guarantor): void {
+		this.guarantors.update((guarantors) => [...guarantors, guarantor]);
+	}
+
+	removeGuarantor(index: number): void {
+		this.guarantors.update((guarantors) => guarantors.filter((_, item) => item !== index));
+	}
+
+	fieldWasChanged(field: string): void {
+		this.fieldChanged.emit(field);
+		if (field === 'tenantIds') this.fieldChanged.emit('tenantIdsDistinct');
+		if (field === 'endDate') this.fieldChanged.emit('endDateOnOrAfterEffectiveDate');
+	}
+
+	showSaveError(): boolean {
+		return (
+			Boolean(this.saveError()) &&
+			(this.saveFieldErrors().length === 0 ||
+				this.saveFieldErrors().some((error) => !this.isInlineServerError(error)))
+		);
+	}
+
+	fieldError(field: AmendmentField): string {
+		const model = this.amendmentModel();
+		const invalid =
+			(field === 'effectiveDate' &&
+				(!model.effectiveDate ||
+					model.effectiveDate < browserToday() ||
+					model.effectiveDate < this.lease().startDate ||
+					model.effectiveDate > this.lease().endDate ||
+					(model.endDate !== '' && model.effectiveDate > model.endDate))) ||
+			(field === 'tenantIds' && !model.tenantIds.length) ||
+			(field === 'endDate' &&
+				(!model.endDate ||
+					model.endDate < this.lease().startDate ||
+					(model.effectiveDate !== '' && model.endDate < model.effectiveDate))) ||
+			(field === 'monthlyRent' && model.monthlyRent <= 0) ||
+			(field === 'monthlyCharges' && model.monthlyCharges < 0) ||
+			(field === 'dueDay' &&
+				(!Number.isInteger(model.dueDay) || model.dueDay < 1 || model.dueDay > 28));
+		if ((!invalid || !this.isTouched(field)) && !this.hasKnownServerError(field)) {
+			return '';
+		}
+		return {
+			effectiveDate:
+				'La prise d’effet doit être comprise entre aujourd’hui et la fin du bail, sans dépasser sa nouvelle fin.',
+			tenantIds: 'Sélectionnez au moins un locataire, sans doublon.',
+			endDate: 'La fin du bail ne peut pas précéder sa prise d’effet ni son début.',
+			monthlyRent: 'Le loyer mensuel doit être strictement positif.',
+			monthlyCharges: 'Les charges mensuelles doivent être positives ou nulles.',
+			dueDay: 'Le jour d’échéance doit être un entier entre 1 et 28.',
+		}[field];
+	}
+
+	private hasKnownServerError(field: AmendmentField): boolean {
+		return this.saveFieldErrors().some(
+			(error) => matchesField(field, error.field) && serverErrorCodes[field].has(error.code),
+		);
+	}
+
+	private isTouched(field: AmendmentField): boolean {
+		switch (field) {
+			case 'effectiveDate':
+				return this.amendmentForm.effectiveDate().touched();
+			case 'tenantIds':
+				return this.amendmentForm.tenantIds().touched();
+			case 'endDate':
+				return this.amendmentForm.endDate().touched();
+			case 'monthlyRent':
+				return this.amendmentForm.monthlyRent().touched();
+			case 'monthlyCharges':
+				return this.amendmentForm.monthlyCharges().touched();
+			case 'dueDay':
+				return this.amendmentForm.dueDay().touched();
+		}
+	}
+
+	private isInlineServerError(error: FieldValidationError): boolean {
+		return amendmentFields.some(
+			(field) => matchesField(field, error.field) && serverErrorCodes[field].has(error.code),
+		);
+	}
+
+	private focusFirstInvalid(): void {
+		const field = amendmentFields.find((item) => this.fieldError(item));
+		this.element.nativeElement.querySelector<HTMLElement>(`#${field ?? ''}`)?.focus();
+	}
+
+	private reset(lease: Lease): void {
+		this.amendmentModel.set({
+			effectiveDate: '',
+			tenantIds: lease.tenantIds,
+			endDate: lease.endDate,
+			monthlyRent: lease.monthlyRent,
+			monthlyCharges: lease.monthlyCharges,
+			dueDay: lease.dueDay,
+			chargeMethod: lease.chargeMethod,
+			indexFlatRate: lease.indexFlatRate,
+		});
+		this.guarantors.set(lease.guarantors);
+	}
+}
+
+function matchesField(field: AmendmentField, path: string): boolean {
+	return (
+		path === field ||
+		(field === 'tenantIds' && path === 'tenantIdsDistinct') ||
+		(field === 'endDate' && path === 'endDateOnOrAfterEffectiveDate')
+	);
+}
+
+function browserToday(): string {
+	const now = new Date();
+	return `${now.getFullYear()}-${String(now.getMonth() + 1).padStart(2, '0')}-${String(now.getDate()).padStart(2, '0')}`;
+}
@@ -177,6 +177,11 @@
 					}
 				</button>
 			} @else if (lease().status === 'ACTIVE' || lease().status === 'ARCHIVED') {
+				@if (lease().status === 'ACTIVE') {
+					<button mat-button type="button" (click)="amendmentRequested.emit(lease())">
+						Préparer un avenant
+					</button>
+				}
 				<button
 					mat-button
 					type="button"
@@ -31,6 +31,7 @@ export class LeaseCard {
 	readonly pdfRequested = output<Lease>();
 	readonly dossierRequested = output<Lease>();
 	readonly editRequested = output<Lease>();
+	readonly amendmentRequested = output<Lease>();
 	readonly finalizationRequested = output<string>();
 	readonly leaseEndRequested = output<string>();
 	readonly inventoryRequested = output<string>();
@@ -0,0 +1,83 @@
+import { FieldValidationError } from '../../services/http-error-message';
+import { LeaseInput } from '../../services/lease.service';
+
+export type LeaseCoreField =
+	| 'apartmentId'
+	| 'tenantIds'
+	| 'leaseType'
+	| 'startDate'
+	| 'monthlyRent'
+	| 'monthlyCharges'
+	| 'securityDeposit'
+	| 'dueDay';
+
+export const leaseCoreFields: LeaseCoreField[] = [
+	'apartmentId',
+	'tenantIds',
+	'leaseType',
+	'startDate',
+	'monthlyRent',
+	'monthlyCharges',
+	'securityDeposit',
+	'dueDay',
+];
+
+const serverErrorCodes: Record<LeaseCoreField, ReadonlySet<string>> = {
+	apartmentId: new Set(['not_blank']),
+	tenantIds: new Set(['not_empty', 'assert_true']),
+	leaseType: new Set(['not_null']),
+	startDate: new Set(['not_null']),
+	monthlyRent: new Set(['positive']),
+	monthlyCharges: new Set(['positive_or_zero']),
+	securityDeposit: new Set(['positive_or_zero']),
+	dueDay: new Set(['min', 'max']),
+};
+
+export function leaseFieldError(
+	model: Pick<LeaseInput, LeaseCoreField>,
+	field: LeaseCoreField,
+	touched: boolean,
+	errors: FieldValidationError[],
+): string {
+	if ((!invalid(model, field) || !touched) && !hasKnownServerError(field, errors)) return '';
+	return {
+		apartmentId: 'Sélectionnez un appartement.',
+		tenantIds: 'Sélectionnez au moins un locataire, sans doublon.',
+		leaseType: 'Sélectionnez un type de bail.',
+		startDate: 'Renseignez la date d’entrée.',
+		monthlyRent: 'Le loyer mensuel doit être strictement positif.',
+		monthlyCharges: 'Les charges mensuelles doivent être positives ou nulles.',
+		securityDeposit: 'Le dépôt de garantie doit être positif ou nul.',
+		dueDay: 'Le jour d’échéance doit être un entier entre 1 et 28.',
+	}[field];
+}
+
+export function isInlineLeaseServerError(error: FieldValidationError): boolean {
+	return leaseCoreFields.some(
+		(field) => matchesField(field, error.field) && serverErrorCodes[field].has(error.code),
+	);
+}
+
+function hasKnownServerError(field: LeaseCoreField, errors: FieldValidationError[]): boolean {
+	return errors.some(
+		(error) => matchesField(field, error.field) && serverErrorCodes[field].has(error.code),
+	);
+}
+
+function matchesField(field: LeaseCoreField, path: string): boolean {
+	return path === field || (field === 'tenantIds' && path === 'tenantIdsDistinct');
+}
+
+function invalid(model: Pick<LeaseInput, LeaseCoreField>, field: LeaseCoreField): boolean {
+	return (
+		(field === 'apartmentId' && !model.apartmentId) ||
+		(field === 'tenantIds' && !model.tenantIds.length) ||
+		(field === 'leaseType' && !model.leaseType) ||
+		(field === 'startDate' && !model.startDate) ||
+		(field === 'monthlyRent' && model.monthlyRent <= 0) ||
+		(field === 'monthlyCharges' && model.monthlyCharges < 0) ||
+		(field === 'securityDeposit' && model.securityDeposit < 0) ||
+		(field === 'dueDay' &&
+			(!Number.isInteger(model.dueDay) || model.dueDay < 1 || model.dueDay > 28))
+	);
+}
@@ -22,7 +22,7 @@
 		(ngSubmit)="save()"
 		[attr.aria-busy]="saving()"
 	>
-		@if (saveError()) {
+		@if (showSaveError()) {
 			<div class="alert-error" role="alert">{{ saveError() }}</div>
 		}
 		<p class="visually-hidden" role="status" aria-live="polite">
@@ -40,23 +40,38 @@
 				<div class="form-grid">
 					<mat-form-field appearance="outline"
 						><mat-label>Appartement</mat-label
-						><mat-select id="apartmentId" [formField]="leaseForm.apartmentId"
+						><mat-select
+							id="apartmentId"
+							[formField]="leaseForm.apartmentId"
+							(selectionChange)="fieldWasChanged('apartmentId')"
+						>
 							><mat-option value="">Sélectionnez un appartement</mat-option>
 							@for (apartment of apartments(); track apartment.id) {
 								<mat-option [value]="apartment.id">{{ apartment.name }}</mat-option>
 							}
-						</mat-select></mat-form-field
-					>
+						</mat-select>
+						@if (fieldError('apartmentId')) {
+							<mat-error>{{ fieldError('apartmentId') }}</mat-error>
+						}
+					</mat-form-field>
 					<mat-form-field appearance="outline"
 						><mat-label>Locataires</mat-label
-						><mat-select id="tenantIds" [formField]="leaseForm.tenantIds" multiple>
+						><mat-select
+							id="tenantIds"
+							[formField]="leaseForm.tenantIds"
+							multiple
+							(selectionChange)="fieldWasChanged('tenantIds')"
+						>
 							@for (tenant of tenants(); track tenant.id) {
 								<mat-option [value]="tenant.id"
 									>{{ tenant.firstName }} {{ tenant.lastName }}</mat-option
 								>
 							}
-						</mat-select></mat-form-field
-					>
+						</mat-select>
+						@if (fieldError('tenantIds')) {
+							<mat-error>{{ fieldError('tenantIds') }}</mat-error>
+						}
+					</mat-form-field>
 				</div>
 			</section>
 			<section class="form-section">
@@ -70,11 +85,18 @@
 				<div class="form-grid three-columns">
 					<mat-form-field appearance="outline"
 						><mat-label>Type de bail</mat-label
-						><mat-select id="leaseType" [formField]="leaseForm.leaseType">
+						><mat-select
+							id="leaseType"
+							[formField]="leaseForm.leaseType"
+							(selectionChange)="fieldWasChanged('leaseType')"
+						>
 							<mat-option value="STANDARD">Location meublée classique</mat-option>
 							<mat-option value="STUDENT">Location meublée étudiante</mat-option>
-						</mat-select></mat-form-field
-					>
+						</mat-select>
+						@if (fieldError('leaseType')) {
+							<mat-error>{{ fieldError('leaseType') }}</mat-error>
+						}
+					</mat-form-field>
 					<mat-form-field appearance="outline"
 						><mat-label>Date d'entrée</mat-label
 						><input
@@ -82,9 +104,10 @@
 							type="date"
 							id="startDate"
 							[formField]="leaseForm.startDate"
+							(change)="fieldWasChanged('startDate')"
 						/>
-						@if (leaseForm.startDate().touched() && leaseForm.startDate().invalid()) {
-							<mat-error>{{ leaseForm.startDate().errors()[0]?.message }}</mat-error>
+						@if (fieldError('startDate')) {
+							<mat-error>{{ fieldError('startDate') }}</mat-error>
 						}
 					</mat-form-field>
 					<div class="lease-duration" aria-live="polite">
@@ -94,6 +117,9 @@
 						</output>
 						<small>{{ leaseDuration().renewalLabel }}</small>
 					</div>
+					@if (overlapWarning()) {
+						<p class="lease-warning" role="note">{{ overlapWarning() }}</p>
+					}
 					<mat-form-field appearance="outline"
 						><mat-label>Loyer mensuel</mat-label
 						><input
@@ -101,8 +127,13 @@
 							type="number"
 							id="monthlyRent"
 							[formField]="leaseForm.monthlyRent"
-						/><span matTextSuffix>€</span></mat-form-field
-					>
+							step="0.01"
+							(change)="fieldWasChanged('monthlyRent')"
+						/><span matTextSuffix>€</span>
+						@if (fieldError('monthlyRent')) {
+							<mat-error>{{ fieldError('monthlyRent') }}</mat-error>
+						}
+					</mat-form-field>
 					<mat-form-field appearance="outline"
 						><mat-label>Charges mensuelles</mat-label
 						><input
@@ -110,8 +141,13 @@
 							type="number"
 							id="monthlyCharges"
 							[formField]="leaseForm.monthlyCharges"
-						/><span matTextSuffix>€</span></mat-form-field
-					>
+							step="0.01"
+							(change)="fieldWasChanged('monthlyCharges')"
+						/><span matTextSuffix>€</span>
+						@if (fieldError('monthlyCharges')) {
+							<mat-error>{{ fieldError('monthlyCharges') }}</mat-error>
+						}
+					</mat-form-field>
 					<mat-form-field appearance="outline"
 						><mat-label>Dépôt de garantie</mat-label
 						><input
@@ -119,11 +155,20 @@
 							type="number"
 							id="securityDeposit"
 							[formField]="leaseForm.securityDeposit"
-						/><span matTextSuffix>€</span></mat-form-field
-					>
+							step="0.01"
+							(change)="fieldWasChanged('securityDeposit')"
+						/><span matTextSuffix>€</span>
+						@if (fieldError('securityDeposit')) {
+							<mat-error>{{ fieldError('securityDeposit') }}</mat-error>
+						}
+					</mat-form-field>
 					<mat-form-field appearance="outline"
 						><mat-label>Mode de récupération des charges</mat-label
-						><mat-select id="chargeMethod" [formField]="leaseForm.chargeMethod">
+						><mat-select
+							id="chargeMethod"
+							[formField]="leaseForm.chargeMethod"
+							(selectionChange)="fieldWasChanged('chargeMethod')"
+						>
 							<mat-option value="FLAT_RATE">Forfait de charges</mat-option>
 							<mat-option value="PROVISION"
 								>Provision avec régularisation annuelle</mat-option
@@ -137,8 +182,13 @@
 							type="number"
 							id="dueDay"
 							[formField]="leaseForm.dueDay"
-						/><mat-hint>Entre le 1er et le 28 du mois</mat-hint></mat-form-field
-					>
+							step="1"
+							(change)="fieldWasChanged('dueDay')"
+						/><mat-hint>Entre le 1er et le 28 du mois</mat-hint>
+						@if (fieldError('dueDay')) {
+							<mat-error>{{ fieldError('dueDay') }}</mat-error>
+						}
+					</mat-form-field>
 					@if (leaseModel().chargeMethod === 'FLAT_RATE') {
 						<mat-checkbox id="indexFlatRate" [formField]="leaseForm.indexFlatRate">
 							Indexer le forfait de charges avec le loyer
@@ -51,7 +51,7 @@ describe('LeaseForm', () => {
 		expect(fixture.nativeElement.textContent).toContain('reconductible tacitement');
 	});

-	it('allows overlapping draft dates but leaves active dates unavailable', () => {
+	it('warns about an active overlap without blocking the draft save', () => {
 		TestBed.configureTestingModule({ providers: [provideNativeDateAdapter()] });
 		const fixture = TestBed.createComponent(LeaseForm);
 		fixture.componentRef.setInput('apartments', []);
@@ -92,13 +92,115 @@ describe('LeaseForm', () => {
 				status: 'ACTIVE',
 			},
 		]);
+		fixture.componentInstance.leaseModel.set({
+			apartmentId: 'apartment-1',
+			tenantIds: ['tenant-1'],
+			leaseType: 'STANDARD',
+			startDate: '2026-02-15',
+			monthlyRent: 800,
+			monthlyCharges: 0,
+			securityDeposit: 0,
+			dueDay: 5,
+			chargeMethod: 'FLAT_RATE',
+			indexFlatRate: false,
+			legalTerms: emptyLeaseLegalTerms(),
+		});
+		const submitted = vi.fn();
+		fixture.componentInstance.submitted.subscribe(submitted);
+		fixture.detectChanges();
+
+		expect(fixture.componentInstance.overlapWarning()).toContain('chevauche');
+		fixture.componentInstance.save();
+		expect(submitted).toHaveBeenCalled();
+	});
+
+	it('accepts centime rent and zero charges or deposit, but blocks negative amounts', () => {
+		TestBed.configureTestingModule({ providers: [provideNativeDateAdapter()] });
+		const fixture = TestBed.createComponent(LeaseForm);
+		fixture.componentRef.setInput('apartments', []);
+		fixture.componentRef.setInput('tenants', []);
+		fixture.componentInstance.leaseModel.set({
+			apartmentId: 'apartment-1',
+			tenantIds: ['tenant-1'],
+			leaseType: 'STANDARD',
+			startDate: '2026-09-01',
+			monthlyRent: 0.01,
+			monthlyCharges: 0,
+			securityDeposit: 0,
+			dueDay: 1,
+			chargeMethod: 'FLAT_RATE',
+			indexFlatRate: false,
+			legalTerms: emptyLeaseLegalTerms(),
+		});
+		const submitted = vi.fn();
+		fixture.componentInstance.submitted.subscribe(submitted);
+		fixture.detectChanges();
+
+		fixture.componentInstance.save();
+		expect(submitted).toHaveBeenCalled();
+
 		fixture.componentInstance.leaseModel.update((lease) => ({
 			...lease,
-			apartmentId: 'apartment-1',
+			monthlyCharges: -0.01,
+			securityDeposit: -1,
 		}));
+		fixture.componentInstance.save();
+		expect(submitted).toHaveBeenCalledTimes(1);
+	});

-		expect(fixture.componentInstance.dateFilter('2026-01-15')).toBe(true);
-		expect(fixture.componentInstance.dateFilter('2026-02-15')).toBe(false);
+	it('keeps values and focuses the first server-invalid core control', async () => {
+		TestBed.configureTestingModule({ providers: [provideNativeDateAdapter()] });
+		const fixture = TestBed.createComponent(LeaseForm);
+		fixture.componentRef.setInput('apartments', []);
+		fixture.componentRef.setInput('tenants', []);
+		fixture.componentInstance.leaseModel.set({
+			apartmentId: 'apartment-1',
+			tenantIds: ['tenant-1'],
+			leaseType: 'STANDARD',
+			startDate: '2026-09-01',
+			monthlyRent: 850,
+			monthlyCharges: 50,
+			securityDeposit: 850,
+			dueDay: 5,
+			chargeMethod: 'FLAT_RATE',
+			indexFlatRate: false,
+			legalTerms: emptyLeaseLegalTerms(),
+		});
+		fixture.detectChanges();
+		fixture.componentRef.setInput('saveError', 'Veuillez corriger les champs signalés.');
+		fixture.componentRef.setInput('saveFieldErrors', [
+			{ field: 'monthlyCharges', code: 'positive_or_zero' },
+			{ field: 'dueDay', code: 'max' },
+		]);
+		fixture.detectChanges();
+		await new Promise<void>((resolve) => queueMicrotask(resolve));
+
+		expect(fixture.componentInstance.leaseModel().monthlyCharges).toBe(50);
+		expect(fixture.nativeElement.textContent).toContain('charges mensuelles doivent être');
+		expect(document.activeElement?.id).toBe('monthlyCharges');
+
+		fixture.componentRef.setInput('saveFieldErrors', [{ field: 'dueDay', code: 'max' }]);
+		fixture.detectChanges();
+		await new Promise<void>((resolve) => queueMicrotask(resolve));
+		expect(document.activeElement?.id).toBe('dueDay');
+	});
+
+	it('maps and clears the duplicate-tenant validation alias through the tenant control', () => {
+		TestBed.configureTestingModule({ providers: [provideNativeDateAdapter()] });
+		const fixture = TestBed.createComponent(LeaseForm);
+		fixture.componentRef.setInput('apartments', []);
+		fixture.componentRef.setInput('tenants', []);
+		fixture.componentRef.setInput('saveFieldErrors', [
+			{ field: 'tenantIdsDistinct', code: 'assert_true' },
+		]);
+		const changed = vi.fn();
+		fixture.componentInstance.fieldChanged.subscribe(changed);
+		fixture.detectChanges();
+
+		expect(fixture.componentInstance.fieldError('tenantIds')).toContain('sans doublon');
+		fixture.componentInstance.fieldWasChanged('tenantIds');
+		expect(changed).toHaveBeenCalledWith('tenantIds');
+		expect(changed).toHaveBeenCalledWith('tenantIdsDistinct');
 	});

 	it('loads a draft for editing', () => {
@@ -1,4 +1,14 @@
-import { Component, computed, effect, input, output, signal, viewChild } from '@angular/core';
+import {
+	Component,
+	computed,
+	effect,
+	ElementRef,
+	inject,
+	input,
+	output,
+	signal,
+	viewChild,
+} from '@angular/core';
 import { FormsModule } from '@angular/forms';
 import { FormRoot, FormField, form, max, min, required, validate } from '@angular/forms/signals';
 import { MatButtonModule } from '@angular/material/button';
@@ -23,6 +33,13 @@ import { GuarantorForm } from '../guarantor-form/guarantor-form';
 import { LeaseClauseEditor } from '../lease-clause-editor/lease-clause-editor';
 import { LeaseLegalTermsEditor } from '../lease-legal-terms/lease-legal-terms';
 import { normalizeLeaseLegalTerms } from '../../services/lease-legal-terms';
+import { FieldValidationError } from '../../services/http-error-message';
+import {
+	isInlineLeaseServerError,
+	leaseCoreFields,
+	leaseFieldError,
+	LeaseCoreField,
+} from './lease-form-validation';

 const frenchDate = new Intl.DateTimeFormat('fr-FR', {
 	day: 'numeric',
@@ -51,14 +68,17 @@ const frenchDate = new Intl.DateTimeFormat('fr-FR', {
 	styleUrl: './lease-form.css',
 })
 export class LeaseForm {
+	private readonly element = inject<ElementRef<HTMLElement>>(ElementRef);
 	readonly apartments = input.required<Apartment[]>();
 	readonly tenants = input.required<Tenant[]>();
 	readonly leases = input<Lease[]>([]);
 	readonly lease = input<Lease>();
 	readonly saving = input(false);
 	readonly saveError = input('');
+	readonly saveFieldErrors = input<FieldValidationError[]>([]);
 	readonly cancelled = output<void>();
 	readonly submitted = output<LeaseInput>();
+	readonly fieldChanged = output<string>();
 	readonly clauseEditor = viewChild(LeaseClauseEditor);
 	readonly legalTermsEditor = viewChild(LeaseLegalTermsEditor);

@@ -81,21 +101,20 @@ export class LeaseForm {
 		required(f.tenantIds);
 		required(f.leaseType);
 		required(f.startDate);
-		validate(f.startDate, ({ value }) =>
-			!value() || this.dateFilter(value())
-				? null
-				: {
-						kind: 'occupied-period',
-						message: 'Cette date est comprise dans la période d’un bail déjà finalisé.',
-					},
-		);
 		required(f.monthlyRent);
-		min(f.monthlyRent, 1);
+		min(f.monthlyRent, Number.MIN_VALUE);
 		required(f.monthlyCharges);
+		min(f.monthlyCharges, 0);
 		required(f.securityDeposit);
+		min(f.securityDeposit, 0);
 		required(f.dueDay);
 		min(f.dueDay, 1);
 		max(f.dueDay, 28);
+		validate(f.dueDay, ({ value }) =>
+			Number.isInteger(value())
+				? null
+				: { kind: 'whole-day', message: 'Le jour d’échéance doit être un nombre entier.' },
+		);
 	});

 	readonly guarantors = signal<Guarantor[]>([]);
@@ -122,6 +141,23 @@ export class LeaseForm {
 					: 'Bail étudiant de neuf mois, non reconductible.',
 		};
 	});
+	readonly overlapWarning = computed(() => {
+		const { apartmentId, startDate } = this.leaseModel();
+		const endDate = this.leaseDuration().endDate;
+		if (!apartmentId || !startDate || !endDate) return '';
+		const overlappingLease = this.leases().find(
+			(lease) =>
+				lease.id !== this.lease()?.id &&
+				lease.status !== 'DRAFT' &&
+				lease.apartmentId === apartmentId &&
+				startDate <= lease.endDate &&
+				endDate >= lease.startDate,
+		);
+		return overlappingLease
+			? 'Cette période chevauche un bail déjà finalisé. Vérifiez la disponibilité avant de finaliser ce brouillon.'
+			: '';
+	});
+	private previousServerErrorSignature = '';

 	constructor() {
 		effect(() => {
@@ -143,20 +179,19 @@ export class LeaseForm {
 			this.guarantors.set(lease.guarantors);
 			this.specialClauses.set(lease.specialClauses ?? []);
 		});
+		effect(() => {
+			const errors = this.saveFieldErrors();
+			const signature = errors
+				.map((error) => `${error.field}:${error.code}`)
+				.sort()
+				.join('|');
+			if (signature && signature !== this.previousServerErrorSignature) {
+				queueMicrotask(() => this.focusFirstInvalid());
+			}
+			this.previousServerErrorSignature = signature;
+		});
 	}

-	readonly dateFilter = (date: string): boolean => {
-		const aptId = this.leaseModel().apartmentId;
-		if (!aptId) return true;
-		return !this.leases().some(
-			(lease) =>
-				lease.status !== 'DRAFT' &&
-				lease.apartmentId === aptId &&
-				date >= lease.startDate &&
-				date <= lease.endDate,
-		);
-	};
-
 	addGuarantor(guarantor: Guarantor): void {
 		this.guarantors.update((guarantors) => [...guarantors, guarantor]);
 	}
@@ -175,6 +210,7 @@ export class LeaseForm {
 			this.legalTermsEditor()?.validate() === false ||
 			this.clauseEditor()?.validate() === false
 		) {
+			queueMicrotask(() => this.focusFirstInvalid());
 			return;
 		}
 		const modelVal = this.leaseModel();
@@ -196,6 +232,59 @@ export class LeaseForm {
 		});
 	}

+	fieldWasChanged(field: string): void {
+		this.fieldChanged.emit(field);
+		if (field === 'tenantIds') this.fieldChanged.emit('tenantIdsDistinct');
+	}
+
+	showSaveError(): boolean {
+		return (
+			Boolean(this.saveError()) &&
+			(this.saveFieldErrors().length === 0 ||
+				this.saveFieldErrors().some((error) => !this.isInlineServerError(error)))
+		);
+	}
+
+	fieldError(field: LeaseCoreField): string {
+		return leaseFieldError(
+			this.leaseModel(),
+			field,
+			this.isTouched(field),
+			this.saveFieldErrors(),
+		);
+	}
+
+	private isTouched(field: LeaseCoreField): boolean {
+		switch (field) {
+			case 'apartmentId':
+				return this.leaseForm.apartmentId().touched();
+			case 'tenantIds':
+				return this.leaseForm.tenantIds().touched();
+			case 'leaseType':
+				return this.leaseForm.leaseType().touched();
+			case 'startDate':
+				return this.leaseForm.startDate().touched();
+			case 'monthlyRent':
+				return this.leaseForm.monthlyRent().touched();
+			case 'monthlyCharges':
+				return this.leaseForm.monthlyCharges().touched();
+			case 'securityDeposit':
+				return this.leaseForm.securityDeposit().touched();
+			case 'dueDay':
+				return this.leaseForm.dueDay().touched();
+		}
+	}
+
+	private isInlineServerError(error: FieldValidationError): boolean {
+		return isInlineLeaseServerError(error);
+	}
+
+	private focusFirstInvalid(): void {
+		const field = leaseCoreFields.find((item) => this.fieldError(item));
+		const control = field && this.element.nativeElement.querySelector<HTMLElement>(`#${field}`);
+		control?.focus();
+	}
+
 	private formatCivilDate(date: string): string {
 		const [year, month, day] = date.split('-').map(Number);
 		return frenchDate.format(new Date(Date.UTC(year, month - 1, day)));
@@ -1,4 +1,4 @@
-@if (!showForm()) {
+@if (!showForm() && !amendingLease()) {
 	<section class="page-section">
 		<header class="page-heading">
 			<div>
@@ -85,6 +85,7 @@
 								(pdfRequested)="workspace.downloadLeasePdf($event)"
 								(dossierRequested)="workspace.downloadLeaseDossier($event)"
 								(editRequested)="openForm($event)"
+								(amendmentRequested)="openAmendment($event)"
 								(finalizationRequested)="openFinalization($event)"
 								(leaseEndRequested)="openLeaseEnd($event)"
 								(inventoryRequested)="openInventory($event)"
@@ -97,7 +98,7 @@
 			}
 		</div>
 	</section>
-} @else {
+} @else if (showForm()) {
 	<app-lease-form
 		[apartments]="workspace.apartments()"
 		[tenants]="workspace.tenants()"
@@ -105,7 +106,20 @@
 		[lease]="editingLease() ?? undefined"
 		[saving]="workspace.leaseSaving(editingLease()?.id)"
 		[saveError]="workspace.leaseSaveError(editingLease()?.id)"
+		[saveFieldErrors]="workspace.leaseSaveFieldErrors(editingLease()?.id)"
 		(cancelled)="closeForm()"
+		(fieldChanged)="workspace.clearLeaseSaveFieldError($event, editingLease()?.id)"
 		(submitted)="saveLease($event)"
 	/>
+} @else if (amendingLease(); as lease) {
+	<app-lease-amendment-form
+		[lease]="lease"
+		[tenants]="workspace.tenants()"
+		[saving]="workspace.leaseSaving(lease.id)"
+		[saveError]="workspace.leaseSaveError(lease.id)"
+		[saveFieldErrors]="workspace.leaseSaveFieldErrors(lease.id)"
+		(cancelled)="closeForm()"
+		(fieldChanged)="workspace.clearLeaseSaveFieldError($event, lease.id)"
+		(submitted)="saveAmendment($event)"
+	/>
 }
@@ -22,12 +22,15 @@ describe('LeasesPage', () => {
 		loadLeases: vi.fn(),
 		loadRentDues: vi.fn(),
 		saveLease: vi.fn(),
+		amendLease: vi.fn(),
 		deleteLease: vi.fn(),
 		downloadLeasePdf: vi.fn(),
 		downloadLeaseDossier: vi.fn(),
 		leaseSaving: vi.fn(() => false),
 		leaseSaveError: vi.fn(() => ''),
+		leaseSaveFieldErrors: vi.fn(() => []),
 		clearLeaseSaveError: vi.fn(),
+		clearLeaseSaveFieldError: vi.fn(),
 		leaseDownloading: vi.fn(() => false),
 		leaseDownloadError: vi.fn(() => ''),
 		leaseRevision: vi.fn(),
@@ -3,13 +3,14 @@ import { Router } from '@angular/router';
 import { MatButtonModule } from '@angular/material/button';
 import { MatProgressBarModule } from '@angular/material/progress-bar';
 import { WorkspaceService } from '../../services/workspace.service';
-import { Lease, LeaseInput, RentDue } from '../../services/lease.service';
+import { Lease, LeaseAmendmentInput, LeaseInput, RentDue } from '../../services/lease.service';
+import { LeaseAmendmentForm } from '../lease-amendment-form/lease-amendment-form';
 import { LeaseCard } from '../lease-card/lease-card';
 import { LeaseForm } from '../lease-form/lease-form';

@Component({
 	selector: 'app-leases-page',
-	imports: [MatButtonModule, MatProgressBarModule, LeaseCard, LeaseForm],
+	imports: [MatButtonModule, MatProgressBarModule, LeaseCard, LeaseForm, LeaseAmendmentForm],
 	templateUrl: './leases-page.html',
 	styleUrl: './leases-page.css',
 })
@@ -18,6 +19,7 @@ export class LeasesPage {
 	private readonly router = inject(Router);
 	readonly showForm = signal(false);
 	readonly editingLease = signal<Lease | null>(null);
+	readonly amendingLease = signal<Lease | null>(null);
 	readonly rentDuesByLease = computed(() => {
 		const rentDuesByLease = new Map<string, RentDue[]>();
 		for (const rentDue of this.workspace.rentDues()) {
@@ -57,6 +59,7 @@ export class LeasesPage {
 	}

 	openForm(lease: Lease | null = null): void {
+		this.amendingLease.set(null);
 		this.editingLease.set(lease);
 		this.workspace.clearLeaseSaveError(lease?.id);
 		this.showForm.set(true);
@@ -64,11 +67,25 @@ export class LeasesPage {
 	closeForm(): void {
 		this.showForm.set(false);
 		this.editingLease.set(null);
+		this.amendingLease.set(null);
 	}
 	saveLease(lease: LeaseInput): void {
 		this.workspace.saveLease(lease, this.editingLease()?.id, () => this.closeForm());
 	}

+	openAmendment(lease: Lease): void {
+		this.showForm.set(false);
+		this.editingLease.set(null);
+		this.workspace.clearLeaseSaveError(lease.id);
+		this.amendingLease.set(lease);
+	}
+
+	saveAmendment(amendment: LeaseAmendmentInput): void {
+		const leaseId = this.amendingLease()?.id;
+		if (!leaseId) return;
+		this.workspace.amendLease(leaseId, amendment, () => this.closeForm());
+	}
+
 	rentDuesFor(lease: Lease): RentDue[] {
 		return lease.id ? (this.rentDuesByLease().get(lease.id) ?? []) : [];
 	}
@@ -2,7 +2,12 @@ import { TestBed } from '@angular/core/testing';
 import { HttpClient } from '@angular/common/http';
 import { of } from 'rxjs';
 import { vi } from 'vitest';
-import { emptyLeaseLegalTerms, LeaseInput, LeaseService } from './lease.service';
+import {
+	emptyLeaseLegalTerms,
+	LeaseAmendmentInput,
+	LeaseInput,
+	LeaseService,
+} from './lease.service';

 describe('LeaseService', () => {
 	const http = {
@@ -43,6 +48,19 @@ describe('LeaseService', () => {
 		service.refreshRentDues().subscribe();
 		service.createLease(lease).subscribe();
 		service.updateLease('lease-1', lease).subscribe();
+		const amendment: LeaseAmendmentInput = {
+			effectiveDate: '2026-10-01',
+			timeZone: 'Europe/Paris',
+			tenantIds: ['tenant-1'],
+			guarantors: [],
+			endDate: '2027-08-31',
+			monthlyRent: 900,
+			monthlyCharges: 50,
+			dueDay: 5,
+			chargeMethod: 'FLAT_RATE',
+			indexFlatRate: false,
+		};
+		service.amendLease('lease-1', amendment).subscribe();
 		service.deleteLease('lease-1').subscribe();
 		service.downloadPdf('lease-1').subscribe();
 		service.createRevision('lease-1').subscribe();
@@ -88,6 +106,7 @@ describe('LeaseService', () => {
 		expect(http.get).not.toHaveBeenCalledWith('/api/rent-dues');
 		expect(http.post).toHaveBeenCalledWith('/api/leases', lease);
 		expect(http.put).toHaveBeenCalledWith('/api/leases/lease-1', lease);
+		expect(http.post).toHaveBeenCalledWith('/api/leases/lease-1/amendments', amendment);
 		expect(http.delete).toHaveBeenCalledWith('/api/leases/lease-1');
 		expect(http.get).toHaveBeenCalledWith('/api/leases/lease-1/pdf', {
 			responseType: 'blob',
@@ -134,6 +134,19 @@ export interface Lease extends LeaseInput {
 	status: 'DRAFT' | 'ACTIVE' | 'ARCHIVED';
 }

+export interface LeaseAmendmentInput {
+	effectiveDate: string;
+	timeZone: string;
+	tenantIds: string[];
+	guarantors: Guarantor[];
+	endDate: string;
+	monthlyRent: number;
+	monthlyCharges: number;
+	dueDay: number;
+	chargeMethod: ChargeMethod;
+	indexFlatRate: boolean;
+}
+
 export interface RentDue {
 	id: string;
 	leaseId: string;
@@ -317,6 +330,10 @@ export class LeaseService {
 		return this.http.put<Lease>(`/api/leases/${id}`, lease);
 	}

+	amendLease(id: string, amendment: LeaseAmendmentInput): Observable<Lease> {
+		return this.http.post<Lease>(`/api/leases/${id}/amendments`, amendment);
+	}
+
 	deleteLease(id: string): Observable<void> {
 		return this.http.delete<void>(`/api/leases/${id}`);
 	}
@@ -7,7 +7,14 @@ import {
 import { Apartment, ApartmentService } from './apartment.service';
 import { FileDownloadService } from './file-download.service';
 import { map, switchMap, tap } from 'rxjs';
-import { Lease, LeaseInput, LeaseRevisionMetadata, LeaseService, RentDue } from './lease.service';
+import {
+	Lease,
+	LeaseAmendmentInput,
+	LeaseInput,
+	LeaseRevisionMetadata,
+	LeaseService,
+	RentDue,
+} from './lease.service';
 import { RentPayment, RentPaymentService } from './rent-payment.service';
 import { Tenant, TenantAttachment, TenantService } from './tenant.service';
 import { TenantAttachmentTransferService } from './tenant-attachment-transfer.service';
@@ -255,6 +262,14 @@ export class WorkspaceService {
 		);
 	}

+	amendLease(id: string, amendment: LeaseAmendmentInput, onSuccess?: () => void): void {
+		this.leaseResource.save(
+			this.resourceKey(id),
+			this.leaseService.amendLease(id, amendment),
+			onSuccess,
+		);
+	}
+
 	leaseSaving(id?: string): boolean {
 		return this.leaseResource.savingKeys().has(this.resourceKey(id));
 	}
@@ -263,10 +278,18 @@ export class WorkspaceService {
 		return this.leaseResource.saveErrors()[this.resourceKey(id)] ?? '';
 	}

+	leaseSaveFieldErrors(id?: string): FieldValidationError[] {
+		return this.leaseResource.saveFieldErrors()[this.resourceKey(id)] ?? [];
+	}
+
 	clearLeaseSaveError(id?: string): void {
 		this.leaseResource.clearSaveError(this.resourceKey(id));
 	}

+	clearLeaseSaveFieldError(field: string, id?: string): void {
+		this.leaseResource.clearSaveFieldError(this.resourceKey(id), field);
+	}
+
 	deleteLease(id: string): void {
 		this.requestConfirmation(
 			{