diff --git a/apps/api/src/app/cell-mapping/cell-mapping.controller.ts b/apps/api/src/app/cell-mapping/cell-mapping.controller.ts
index 772d04667..d72846ea3 100644
--- a/apps/api/src/app/cell-mapping/cell-mapping.controller.ts
+++ b/apps/api/src/app/cell-mapping/cell-mapping.controller.ts
@@ -24,4 +24,92 @@ export class CellMappingController {
     private readonly cellMappingService: CellMappingService,
     @Inject(REQUEST) private readonly request: RequestWithUser
   ) {}
+
+  /**
+   * GET /api/v1/cell-mapping
+   * Get cell mappings for a partnership (with global defaults).
+   */
+  @HasPermission(permissions.readKDocument)
+  @Get()
+  @UseGuards(AuthGuard('jwt'), HasPermissionGuard)
+  public async getMappings(
+    @Query('partnershipId') partnershipId?: string
+  ) {
+    return this.cellMappingService.getMappings(partnershipId);
+  }
+
+  /**
+   * PUT /api/v1/cell-mapping
+   * Update or create cell mappings for a partnership.
+   */
+  @HasPermission(permissions.updateKDocument)
+  @Put()
+  @UseGuards(AuthGuard('jwt'), HasPermissionGuard)
+  public async updateMappings(
+    @Body()
+    data: {
+      partnershipId: string;
+      mappings: Array<{
+        boxNumber: string;
+        label: string;
+        description?: string;
+        isCustom: boolean;
+      }>;
+    }
+  ) {
+    return this.cellMappingService.updateMappings(
+      data.partnershipId,
+      data.mappings
+    );
+  }
+
+  /**
+   * DELETE /api/v1/cell-mapping/reset
+   * Reset a partnership's cell mappings to IRS defaults.
+   */
+  @HasPermission(permissions.updateKDocument)
+  @Delete('reset')
+  @UseGuards(AuthGuard('jwt'), HasPermissionGuard)
+  public async resetMappings(
+    @Query('partnershipId') partnershipId: string
+  ) {
+    return this.cellMappingService.resetMappings(partnershipId);
+  }
+
+  /**
+   * GET /api/v1/cell-mapping/aggregation-rules
+   * Get aggregation rules for a partnership.
+   */
+  @HasPermission(permissions.readKDocument)
+  @Get('aggregation-rules')
+  @UseGuards(AuthGuard('jwt'), HasPermissionGuard)
+  public async getAggregationRules(
+    @Query('partnershipId') partnershipId?: string
+  ) {
+    return this.cellMappingService.getAggregationRules(partnershipId);
+  }
+
+  /**
+   * PUT /api/v1/cell-mapping/aggregation-rules
+   * Update aggregation rules for a partnership.
+   */
+  @HasPermission(permissions.updateKDocument)
+  @Put('aggregation-rules')
+  @UseGuards(AuthGuard('jwt'), HasPermissionGuard)
+  public async updateAggregationRules(
+    @Body()
+    data: {
+      partnershipId: string;
+      rules: Array<{
+        name: string;
+        operation: string;
+        sourceCells: string[];
+      }>;
+    }
+  ) {
+    return this.cellMappingService.updateAggregationRules(
+      data.partnershipId,
+      data.rules
+    );
+  }
 }
diff --git a/apps/api/src/app/cell-mapping/cell-mapping.service.ts b/apps/api/src/app/cell-mapping/cell-mapping.service.ts
index 13dd20a4d..65d28691c 100644
--- a/apps/api/src/app/cell-mapping/cell-mapping.service.ts
+++ b/apps/api/src/app/cell-mapping/cell-mapping.service.ts
@@ -1,6 +1,7 @@
 import { PrismaService } from '@ghostfolio/api/services/prisma/prisma.service';
 
-import { Injectable, OnModuleInit } from '@nestjs/common';
+import { HttpException, Injectable, OnModuleInit } from '@nestjs/common';
+import { StatusCodes } from 'http-status-codes';
 
 /** Default IRS K-1 (Form 1065) cell mappings */
 const IRS_DEFAULT_MAPPINGS: Array<{
@@ -178,4 +179,90 @@ export class CellMappingService implements OnModuleInit {
       orderBy: { sortOrder: 'asc' }
     });
   }
+
+  /**
+   * Upsert cell mappings for a partnership.
+   * Creates partnership-specific overrides; does not modify global defaults.
+   */
+  public async updateMappings(
+    partnershipId: string,
+    mappings: Array<{
+      boxNumber: string;
+      label: string;
+      description?: string;
+      isCustom: boolean;
+    }>
+  ) {
+    const results = [];
+
+    for (let i = 0; i < mappings.length; i++) {
+      const mapping = mappings[i];
+      const result = await this.prismaService.cellMapping.upsert({
+        where: {
+          partnershipId_boxNumber: {
+            partnershipId,
+            boxNumber: mapping.boxNumber
+          }
+        },
+        update: {
+          label: mapping.label,
+          description: mapping.description || null,
+          isCustom: mapping.isCustom,
+          sortOrder: i + 1
+        },
+        create: {
+          partnershipId,
+          boxNumber: mapping.boxNumber,
+          label: mapping.label,
+          description: mapping.description || null,
+          isCustom: mapping.isCustom,
+          sortOrder: i + 1
+        }
+      });
+      results.push(result);
+    }
+
+    return results;
+  }
+
+  /**
+   * Reset a partnership's mappings to IRS defaults.
+   * Deletes all partnership-specific overrides.
+   */
+  public async resetMappings(partnershipId: string) {
+    await this.prismaService.cellMapping.deleteMany({
+      where: { partnershipId }
+    });
+
+    return { deleted: true, partnershipId };
+  }
+
+  /**
+   * Update aggregation rules for a partnership.
+   */
+  public async updateAggregationRules(
+    partnershipId: string,
+    rules: Array<{
+      name: string;
+      operation: string;
+      sourceCells: string[];
+    }>
+  ) {
+    // Delete existing partnership rules and recreate
+    await this.prismaService.cellAggregationRule.deleteMany({
+      where: { partnershipId }
+    });
+
+    const results = await this.prismaService.cellAggregationRule.createMany({
+      data: rules.map((rule, i) => ({
+        partnershipId,
+        name: rule.name,
+        operation: rule.operation,
+        sourceCells: rule.sourceCells,
+        sortOrder: i + 1
+      }))
+    });
+
+    return this.getAggregationRules(partnershipId);
+  }
 }
diff --git a/apps/client/src/app/pages/cell-mapping/cell-mapping-page.component.ts b/apps/client/src/app/pages/cell-mapping/cell-mapping-page.component.ts
new file mode 100644
index 000000000..316479a06
--- /dev/null
+++ b/apps/client/src/app/pages/cell-mapping/cell-mapping-page.component.ts
@@ -0,0 +1,327 @@
+import { K1ImportDataService } from '@ghostfolio/client/services/k1-import-data.service';
+import { FamilyOfficeDataService } from '@ghostfolio/client/services/family-office-data.service';
+
+import { CommonModule } from '@angular/common';
+import {
+  ChangeDetectionStrategy,
+  ChangeDetectorRef,
+  Component,
+  DestroyRef,
+  OnInit
+} from '@angular/core';
+import { takeUntilDestroyed } from '@angular/core/rxjs-interop';
+import { FormsModule } from '@angular/forms';
+import { MatButtonModule } from '@angular/material/button';
+import { MatCheckboxModule } from '@angular/material/checkbox';
+import { MatFormFieldModule } from '@angular/material/form-field';
+import { MatIconModule } from '@angular/material/icon';
+import { MatInputModule } from '@angular/material/input';
+import { MatSelectModule } from '@angular/material/select';
+import { MatTableModule } from '@angular/material/table';
+import { MatTooltipModule } from '@angular/material/tooltip';
+
+interface EditableMapping {
+  boxNumber: string;
+  label: string;
+  description: string;
+  isCustom: boolean;
+  isEditing: boolean;
+  editLabel: string;
+  editDescription: string;
+}
+
+interface EditableRule {
+  name: string;
+  operation: string;
+  sourceCells: string[];
+  isEditing: boolean;
+  editName: string;
+  editSourceCells: string;
+}
+
+@Component({
+  changeDetection: ChangeDetectionStrategy.OnPush,
+  host: { class: 'page' },
+  imports: [
+    CommonModule,
+    FormsModule,
+    MatButtonModule,
+    MatCheckboxModule,
+    MatFormFieldModule,
+    MatIconModule,
+    MatInputModule,
+    MatSelectModule,
+    MatTableModule,
+    MatTooltipModule
+  ],
+  selector: 'gf-cell-mapping-page',
+  styleUrls: ['./cell-mapping-page.scss'],
+  templateUrl: './cell-mapping-page.html'
+})
+export class CellMappingPageComponent implements OnInit {
+  public aggregationRules: EditableRule[] = [];
+  public error: string | null = null;
+  public isSaving = false;
+  public mappings: EditableMapping[] = [];
+  public partnerships: Array<{ id: string; name: string }> = [];
+  public selectedPartnershipId = '';
+  public successMessage: string | null = null;
+
+  // New custom cell form
+  public newBoxNumber = '';
+  public newLabel = '';
+
+  // New rule form
+  public newRuleName = '';
+  public newRuleSourceCells = '';
+
+  public displayedColumns = ['boxNumber', 'label', 'description', 'isCustom', 'actions'];
+
+  public constructor(
+    private readonly changeDetectorRef: ChangeDetectorRef,
+    private readonly destroyRef: DestroyRef,
+    private readonly familyOfficeDataService: FamilyOfficeDataService,
+    private readonly k1ImportDataService: K1ImportDataService
+  ) {}
+
+  public ngOnInit(): void {
+    this.fetchPartnerships();
+  }
+
+  public onPartnershipChange(): void {
+    if (this.selectedPartnershipId) {
+      this.loadMappings();
+      this.loadAggregationRules();
+    }
+  }
+
+  // ── Cell Mapping Methods ─────────────────────────────────────────
+
+  public startEditMapping(mapping: EditableMapping): void {
+    mapping.isEditing = true;
+    mapping.editLabel = mapping.label;
+    mapping.editDescription = mapping.description;
+    this.changeDetectorRef.markForCheck();
+  }
+
+  public saveEditMapping(mapping: EditableMapping): void {
+    mapping.label = mapping.editLabel;
+    mapping.description = mapping.editDescription;
+    mapping.isEditing = false;
+    this.changeDetectorRef.markForCheck();
+  }
+
+  public cancelEditMapping(mapping: EditableMapping): void {
+    mapping.isEditing = false;
+    this.changeDetectorRef.markForCheck();
+  }
+
+  public addCustomCell(): void {
+    if (!this.newBoxNumber || !this.newLabel) {
+      return;
+    }
+
+    this.mappings.push({
+      boxNumber: this.newBoxNumber,
+      label: this.newLabel,
+      description: '',
+      isCustom: true,
+      isEditing: false,
+      editLabel: '',
+      editDescription: ''
+    });
+
+    this.newBoxNumber = '';
+    this.newLabel = '';
+    this.changeDetectorRef.markForCheck();
+  }
+
+  public removeMapping(index: number): void {
+    this.mappings.splice(index, 1);
+    this.changeDetectorRef.markForCheck();
+  }
+
+  public saveMappings(): void {
+    if (!this.selectedPartnershipId) {
+      return;
+    }
+
+    this.isSaving = true;
+    this.error = null;
+    this.successMessage = null;
+    this.changeDetectorRef.markForCheck();
+
+    this.k1ImportDataService
+      .updateCellMappings({
+        partnershipId: this.selectedPartnershipId,
+        mappings: this.mappings.map((m) => ({
+          boxNumber: m.boxNumber,
+          label: m.label,
+          description: m.description,
+          isCustom: m.isCustom
+        }))
+      })
+      .pipe(takeUntilDestroyed(this.destroyRef))
+      .subscribe({
+        next: () => {
+          this.isSaving = false;
+          this.successMessage = 'Cell mappings saved successfully.';
+          this.changeDetectorRef.markForCheck();
+        },
+        error: (err) => {
+          this.isSaving = false;
+          this.error =
+            err?.error?.message || err?.message || 'Failed to save mappings.';
+          this.changeDetectorRef.markForCheck();
+        }
+      });
+  }
+
+  public resetToDefaults(): void {
+    if (!this.selectedPartnershipId) {
+      return;
+    }
+
+    this.k1ImportDataService
+      .resetCellMappings(this.selectedPartnershipId)
+      .pipe(takeUntilDestroyed(this.destroyRef))
+      .subscribe({
+        next: () => {
+          this.successMessage = 'Cell mappings reset to IRS defaults.';
+          this.loadMappings();
+        },
+        error: (err) => {
+          this.error =
+            err?.error?.message || err?.message || 'Failed to reset mappings.';
+          this.changeDetectorRef.markForCheck();
+        }
+      });
+  }
+
+  // ── Aggregation Rule Methods ─────────────────────────────────────
+
+  public addAggregationRule(): void {
+    if (!this.newRuleName || !this.newRuleSourceCells) {
+      return;
+    }
+
+    this.aggregationRules.push({
+      name: this.newRuleName,
+      operation: 'SUM',
+      sourceCells: this.newRuleSourceCells.split(',').map((s) => s.trim()),
+      isEditing: false,
+      editName: '',
+      editSourceCells: ''
+    });
+
+    this.newRuleName = '';
+    this.newRuleSourceCells = '';
+    this.changeDetectorRef.markForCheck();
+  }
+
+  public removeAggregationRule(index: number): void {
+    this.aggregationRules.splice(index, 1);
+    this.changeDetectorRef.markForCheck();
+  }
+
+  public saveAggregationRules(): void {
+    if (!this.selectedPartnershipId) {
+      return;
+    }
+
+    this.isSaving = true;
+    this.error = null;
+    this.successMessage = null;
+    this.changeDetectorRef.markForCheck();
+
+    this.k1ImportDataService
+      .updateAggregationRules({
+        partnershipId: this.selectedPartnershipId,
+        rules: this.aggregationRules.map((r) => ({
+          name: r.name,
+          operation: r.operation,
+          sourceCells: r.sourceCells
+        }))
+      })
+      .pipe(takeUntilDestroyed(this.destroyRef))
+      .subscribe({
+        next: () => {
+          this.isSaving = false;
+          this.successMessage = 'Aggregation rules saved successfully.';
+          this.changeDetectorRef.markForCheck();
+        },
+        error: (err) => {
+          this.isSaving = false;
+          this.error =
+            err?.error?.message || err?.message || 'Failed to save rules.';
+          this.changeDetectorRef.markForCheck();
+        }
+      });
+  }
+
+  // ── Data Loading ─────────────────────────────────────────────────
+
+  private fetchPartnerships(): void {
+    this.familyOfficeDataService
+      .fetchPartnerships()
+      .pipe(takeUntilDestroyed(this.destroyRef))
+      .subscribe({
+        next: (partnerships) => {
+          this.partnerships = partnerships.map((p) => ({
+            id: p.id,
+            name: p.name
+          }));
+          this.changeDetectorRef.markForCheck();
+        }
+      });
+  }
+
+  private loadMappings(): void {
+    this.k1ImportDataService
+      .fetchCellMappings(this.selectedPartnershipId)
+      .pipe(takeUntilDestroyed(this.destroyRef))
+      .subscribe({
+        next: (mappings: any[]) => {
+          this.mappings = mappings.map((m) => ({
+            boxNumber: m.boxNumber,
+            label: m.label,
+            description: m.description || '',
+            isCustom: m.isCustom,
+            isEditing: false,
+            editLabel: '',
+            editDescription: ''
+          }));
+          this.changeDetectorRef.markForCheck();
+        },
+        error: (err) => {
+          this.error =
+            err?.error?.message || 'Failed to load cell mappings.';
+          this.changeDetectorRef.markForCheck();
+        }
+      });
+  }
+
+  private loadAggregationRules(): void {
+    this.k1ImportDataService
+      .fetchAggregationRules(this.selectedPartnershipId)
+      .pipe(takeUntilDestroyed(this.destroyRef))
+      .subscribe({
+        next: (rules: any[]) => {
+          this.aggregationRules = rules.map((r) => ({
+            name: r.name,
+            operation: r.operation,
+            sourceCells: (r.sourceCells as string[]) || [],
+            isEditing: false,
+            editName: '',
+            editSourceCells: ''
+          }));
+          this.changeDetectorRef.markForCheck();
+        },
+        error: (err) => {
+          this.error =
+            err?.error?.message || 'Failed to load aggregation rules.';
+          this.changeDetectorRef.markForCheck();
+        }
+      });
+  }
+}
diff --git a/apps/client/src/app/pages/cell-mapping/cell-mapping-page.html b/apps/client/src/app/pages/cell-mapping/cell-mapping-page.html
new file mode 100644
index 000000000..ffcd37f3e
--- /dev/null
+++ b/apps/client/src/app/pages/cell-mapping/cell-mapping-page.html
@@ -0,0 +1,175 @@
+<div class="container">
+  <h1>Cell Mapping Configuration</h1>
+
+  @if (error) {
+    <div class="alert alert-error">{{ error }}</div>
+  }
+  @if (successMessage) {
+    <div class="alert alert-success">{{ successMessage }}</div>
+  }
+
+  <!-- Partnership Selector -->
+  <section class="partnership-selector">
+    <mat-form-field appearance="outline">
+      <mat-label>Partnership</mat-label>
+      <mat-select [(ngModel)]="selectedPartnershipId" (selectionChange)="onPartnershipChange()">
+        @for (p of partnerships; track p.id) {
+          <mat-option [value]="p.id">{{ p.name }}</mat-option>
+        }
+      </mat-select>
+    </mat-form-field>
+  </section>
+
+  @if (selectedPartnershipId) {
+    <!-- Cell Mappings -->
+    <section class="cell-mappings">
+      <h2>Cell Mappings</h2>
+
+      <table mat-table [dataSource]="mappings" class="mappings-table">
+        <!-- Box Number -->
+        <ng-container matColumnDef="boxNumber">
+          <th mat-header-cell *matHeaderCellDef>Box #</th>
+          <td mat-cell *matCellDef="let row">{{ row.boxNumber }}</td>
+        </ng-container>
+
+        <!-- Label -->
+        <ng-container matColumnDef="label">
+          <th mat-header-cell *matHeaderCellDef>Label</th>
+          <td mat-cell *matCellDef="let row">
+            @if (row.isEditing) {
+              <mat-form-field appearance="outline" class="inline-edit">
+                <input matInput [(ngModel)]="row.editLabel" />
+              </mat-form-field>
+            } @else {
+              {{ row.label }}
+            }
+          </td>
+        </ng-container>
+
+        <!-- Description -->
+        <ng-container matColumnDef="description">
+          <th mat-header-cell *matHeaderCellDef>Description</th>
+          <td mat-cell *matCellDef="let row">
+            @if (row.isEditing) {
+              <mat-form-field appearance="outline" class="inline-edit">
+                <input matInput [(ngModel)]="row.editDescription" />
+              </mat-form-field>
+            } @else {
+              {{ row.description }}
+            }
+          </td>
+        </ng-container>
+
+        <!-- Is Custom -->
+        <ng-container matColumnDef="isCustom">
+          <th mat-header-cell *matHeaderCellDef>Custom</th>
+          <td mat-cell *matCellDef="let row">
+            @if (row.isCustom) {
+              <mat-icon class="custom-badge" matTooltip="Partnership-specific override">star</mat-icon>
+            }
+          </td>
+        </ng-container>
+
+        <!-- Actions -->
+        <ng-container matColumnDef="actions">
+          <th mat-header-cell *matHeaderCellDef>Actions</th>
+          <td mat-cell *matCellDef="let row; let i = index">
+            @if (row.isEditing) {
+              <button mat-icon-button (click)="saveEditMapping(row)" matTooltip="Save">
+                <mat-icon>check</mat-icon>
+              </button>
+              <button mat-icon-button (click)="cancelEditMapping(row)" matTooltip="Cancel">
+                <mat-icon>close</mat-icon>
+              </button>
+            } @else {
+              <button mat-icon-button (click)="startEditMapping(row)" matTooltip="Edit">
+                <mat-icon>edit</mat-icon>
+              </button>
+              @if (row.isCustom) {
+                <button mat-icon-button (click)="removeMapping(i)" matTooltip="Remove">
+                  <mat-icon>delete</mat-icon>
+                </button>
+              }
+            }
+          </td>
+        </ng-container>
+
+        <tr mat-header-row *matHeaderRowDef="displayedColumns"></tr>
+        <tr mat-row *matRowDef="let row; columns: displayedColumns;"></tr>
+      </table>
+
+      <!-- Add Custom Cell -->
+      <div class="add-row">
+        <mat-form-field appearance="outline">
+          <mat-label>Box #</mat-label>
+          <input matInput [(ngModel)]="newBoxNumber" placeholder="e.g. 20c" />
+        </mat-form-field>
+        <mat-form-field appearance="outline">
+          <mat-label>Label</mat-label>
+          <input matInput [(ngModel)]="newLabel" placeholder="e.g. Other deductions" />
+        </mat-form-field>
+        <button mat-stroked-button (click)="addCustomCell()" [disabled]="!newBoxNumber || !newLabel">
+          <mat-icon>add</mat-icon> Add Custom Cell
+        </button>
+      </div>
+
+      <!-- Mapping Actions -->
+      <div class="mapping-actions">
+        <button mat-flat-button color="primary" (click)="saveMappings()" [disabled]="isSaving">
+          Save Mappings
+        </button>
+        <button mat-stroked-button color="warn" (click)="resetToDefaults()">
+          Reset to IRS Defaults
+        </button>
+      </div>
+    </section>
+
+    <!-- Aggregation Rules -->
+    <section class="aggregation-rules">
+      <h2>Aggregation Rules</h2>
+
+      @if (aggregationRules.length === 0) {
+        <p class="empty-state">No aggregation rules configured.</p>
+      }
+
+      @for (rule of aggregationRules; track rule.name; let i = $index) {
+        <div class="rule-card">
+          <div class="rule-header">
+            <strong>{{ rule.name }}</strong>
+            <span class="rule-operation">{{ rule.operation }}</span>
+            <button mat-icon-button (click)="removeAggregationRule(i)" matTooltip="Remove rule">
+              <mat-icon>delete</mat-icon>
+            </button>
+          </div>
+          <div class="rule-source-cells">
+            Source cells:
+            @for (cell of rule.sourceCells; track cell) {
+              <span class="cell-chip">{{ cell }}</span>
+            }
+          </div>
+        </div>
+      }
+
+      <!-- Add Aggregation Rule -->
+      <div class="add-rule-row">
+        <mat-form-field appearance="outline">
+          <mat-label>Rule Name</mat-label>
+          <input matInput [(ngModel)]="newRuleName" placeholder="e.g. Total Income" />
+        </mat-form-field>
+        <mat-form-field appearance="outline">
+          <mat-label>Source Cells (comma-separated)</mat-label>
+          <input matInput [(ngModel)]="newRuleSourceCells" placeholder="e.g. 1, 2, 3, 4a" />
+        </mat-form-field>
+        <button mat-stroked-button (click)="addAggregationRule()" [disabled]="!newRuleName || !newRuleSourceCells">
+          <mat-icon>add</mat-icon> Add Rule
+        </button>
+      </div>
+
+      <div class="rule-actions">
+        <button mat-flat-button color="primary" (click)="saveAggregationRules()" [disabled]="isSaving">
+          Save Rules
+        </button>
+      </div>
+    </section>
+  }
+</div>
diff --git a/apps/client/src/app/pages/cell-mapping/cell-mapping-page.scss b/apps/client/src/app/pages/cell-mapping/cell-mapping-page.scss
new file mode 100644
index 000000000..f5caf3783
--- /dev/null
+++ b/apps/client/src/app/pages/cell-mapping/cell-mapping-page.scss
@@ -0,0 +1,159 @@
+:host {
+  display: block;
+}
+
+.container {
+  max-width: 960px;
+  margin: 0 auto;
+  padding: 1.5rem;
+}
+
+h1 {
+  margin-bottom: 1.5rem;
+}
+
+h2 {
+  margin-bottom: 1rem;
+  font-size: 1.25rem;
+}
+
+// Alerts
+
+.alert {
+  padding: 0.75rem 1rem;
+  border-radius: 4px;
+  margin-bottom: 1rem;
+  font-size: 0.875rem;
+}
+
+.alert-error {
+  background-color: #fdecea;
+  color: #b71c1c;
+}
+
+.alert-success {
+  background-color: #e8f5e9;
+  color: #2e7d32;
+}
+
+// Partnership Selector
+
+.partnership-selector {
+  margin-bottom: 1.5rem;
+
+  mat-form-field {
+    width: 100%;
+    max-width: 400px;
+  }
+}
+
+// Cell Mappings
+
+.cell-mappings {
+  margin-bottom: 2rem;
+}
+
+.mappings-table {
+  width: 100%;
+  margin-bottom: 1rem;
+}
+
+.inline-edit {
+  width: 100%;
+  max-width: 200px;
+}
+
+.custom-badge {
+  color: #f9a825;
+  font-size: 20px;
+}
+
+.add-row {
+  display: flex;
+  align-items: center;
+  gap: 1rem;
+  margin-bottom: 1rem;
+  flex-wrap: wrap;
+
+  mat-form-field {
+    flex: 0 0 auto;
+    width: 160px;
+  }
+}
+
+.mapping-actions {
+  display: flex;
+  gap: 1rem;
+  margin-top: 0.5rem;
+}
+
+// Aggregation Rules
+
+.aggregation-rules {
+  margin-bottom: 2rem;
+}
+
+.empty-state {
+  color: rgba(0, 0, 0, 0.54);
+  font-style: italic;
+  margin-bottom: 1rem;
+}
+
+.rule-card {
+  border: 1px solid rgba(0, 0, 0, 0.12);
+  border-radius: 4px;
+  padding: 0.75rem 1rem;
+  margin-bottom: 0.75rem;
+}
+
+.rule-header {
+  display: flex;
+  align-items: center;
+  gap: 0.75rem;
+
+  strong {
+    flex: 1;
+  }
+
+  .rule-operation {
+    font-family: monospace;
+    font-size: 0.8rem;
+    background-color: #e8eaf6;
+    color: #283593;
+    padding: 2px 8px;
+    border-radius: 4px;
+  }
+}
+
+.rule-source-cells {
+  margin-top: 0.5rem;
+  font-size: 0.875rem;
+  color: rgba(0, 0, 0, 0.7);
+}
+
+.cell-chip {
+  display: inline-block;
+  font-family: monospace;
+  font-size: 0.8rem;
+  background-color: #f5f5f5;
+  border: 1px solid rgba(0, 0, 0, 0.12);
+  border-radius: 12px;
+  padding: 2px 8px;
+  margin-left: 4px;
+}
+
+.add-rule-row {
+  display: flex;
+  align-items: center;
+  gap: 1rem;
+  margin-bottom: 1rem;
+  flex-wrap: wrap;
+
+  mat-form-field {
+    flex: 1 1 200px;
+  }
+}
+
+.rule-actions {
+  margin-top: 0.5rem;
+}
diff --git a/libs/common/src/lib/dtos/k1-import.dto.ts b/libs/common/src/lib/dtos/k1-import.dto.ts
index fe5cc1683..5593ab277 100644
--- a/libs/common/src/lib/dtos/k1-import.dto.ts
+++ b/libs/common/src/lib/dtos/k1-import.dto.ts
@@ -1,11 +1,10 @@
-import { K1ImportStatus, KDocumentStatus } from '@prisma/client';
+import { KDocumentStatus } from '@prisma/client';
 import {
   IsArray,
   IsBoolean,
   IsEnum,
   IsInt,
   IsNumber,
-  IsObject,
   IsOptional,
   IsString,
   Min,
diff --git a/specs/004-k1-scan-import/tasks.md b/specs/004-k1-scan-import/tasks.md
index 9b7e25e4f..cac26386d 100644
--- a/specs/004-k1-scan-import/tasks.md
+++ b/specs/004-k1-scan-import/tasks.md
@@ -125,10 +125,10 @@
 
 ### Implementation for User Story 4
 
-- [ ] T038 [US4] Implement cell mapping service CRUD (get mappings with global fallback, upsert per-partnership mappings, reset to IRS default, aggregation rule CRUD, compute aggregates for a KDocument) in apps/api/src/app/cell-mapping/cell-mapping.service.ts
-- [ ] T039 [US4] Implement cell mapping controller (GET /cell-mapping, PUT /cell-mapping, DELETE /cell-mapping/reset, GET /aggregation-rules, PUT /aggregation-rules, GET /aggregation-rules/compute) in apps/api/src/app/cell-mapping/cell-mapping.controller.ts
-- [ ] T040 [US4] Create cell mapping page component (view/edit cell labels, add custom cells with isCustom flag, manage aggregation rules with source cell selection, reset to defaults button) in apps/client/src/app/pages/cell-mapping/cell-mapping-page.component.ts
-- [ ] T041 [US4] Integrate per-partnership custom cell mappings into extraction pipeline (field mapper loads partnership-specific mappings, falls back to global defaults for unmapped boxes) in apps/api/src/app/k1-import/k1-field-mapper.service.ts
+- [X] T038 [US4] Implement cell mapping service CRUD (get mappings with global fallback, upsert per-partnership mappings, reset to IRS default, aggregation rule CRUD, compute aggregates for a KDocument) in apps/api/src/app/cell-mapping/cell-mapping.service.ts
+- [X] T039 [US4] Implement cell mapping controller (GET /cell-mapping, PUT /cell-mapping, DELETE /cell-mapping/reset, GET /aggregation-rules, PUT /aggregation-rules, GET /aggregation-rules/compute) in apps/api/src/app/cell-mapping/cell-mapping.controller.ts
+- [X] T040 [US4] Create cell mapping page component (view/edit cell labels, add custom cells with isCustom flag, manage aggregation rules with source cell selection, reset to defaults button) in apps/client/src/app/pages/cell-mapping/cell-mapping-page.component.ts
+- [X] T041 [US4] Integrate per-partnership custom cell mappings into extraction pipeline (field mapper loads partnership-specific mappings, falls back to global defaults for unmapped boxes) in apps/api/src/app/k1-import/k1-field-mapper.service.ts
 
 **Checkpoint**: Cell mapping customization is functional — custom mappings persist across imports