diff --git a/apps/beheer/src/app/app.html b/apps/beheer/src/app/app.html
index 0680b43..7031d80 100644
--- a/apps/beheer/src/app/app.html
+++ b/apps/beheer/src/app/app.html
@@ -1 +1,5 @@
+<nav aria-label="Beheer" class="utrecht-theme">
+  <a routerLink="/" routerLinkActive="active" [routerLinkActiveOptions]="{ exact: true }">Catalogus</a>
+  <a routerLink="/default-fill" routerLinkActive="active">Default-fill</a>
+</nav>
 <router-outlet></router-outlet>
diff --git a/apps/beheer/src/app/app.routes.ts b/apps/beheer/src/app/app.routes.ts
index 942b1be..03aa788 100644
--- a/apps/beheer/src/app/app.routes.ts
+++ b/apps/beheer/src/app/app.routes.ts
@@ -1,7 +1,9 @@
 import { Route } from '@angular/router';
 import { authenticatedGuard } from 'auth';
 import { CatalogusPage } from './catalogus/catalogus-page';
+import { DefaultFillPage } from './default-fill/default-fill-page';
 
 export const appRoutes: Route[] = [
   { path: '', component: CatalogusPage, canActivate: [authenticatedGuard] },
+  { path: 'default-fill', component: DefaultFillPage, canActivate: [authenticatedGuard] },
 ];
diff --git a/apps/beheer/src/app/default-fill/default-fill-page.html b/apps/beheer/src/app/default-fill/default-fill-page.html
new file mode 100644
index 0000000..ea2e48e
--- /dev/null
+++ b/apps/beheer/src/app/default-fill/default-fill-page.html
@@ -0,0 +1,60 @@
+<main utrecht-document class="utrecht-theme">
+  <utrecht-article>
+    <utrecht-heading-1>Default-fill</utrecht-heading-1>
+    <p utrecht-paragraph>
+      De ZGW-standaardwaarden die de ACL op elke nieuwe zaak invult (ADR-0003). Een wijziging geldt
+      voor de eerstvolgende zaak.
+    </p>
+
+    @if (loading()) {
+      <p utrecht-paragraph role="status">Bezig met laden…</p>
+    } @else if (loaded()) {
+      <form (submit)="save(); $event.preventDefault()">
+        <p>
+          <label for="bronorganisatie">Bronorganisatie</label><br />
+          <input
+            id="bronorganisatie"
+            name="bronorganisatie"
+            [value]="bronorganisatie()"
+            (input)="bronorganisatie.set($any($event.target).value)"
+          />
+        </p>
+        <p>
+          <label for="verantwoordelijkeOrganisatie">Verantwoordelijke organisatie</label><br />
+          <input
+            id="verantwoordelijkeOrganisatie"
+            name="verantwoordelijkeOrganisatie"
+            [value]="verantwoordelijkeOrganisatie()"
+            (input)="verantwoordelijkeOrganisatie.set($any($event.target).value)"
+          />
+        </p>
+        <p>
+          <label for="vertrouwelijkheidaanduiding">Vertrouwelijkheidaanduiding</label><br />
+          <input
+            id="vertrouwelijkheidaanduiding"
+            name="vertrouwelijkheidaanduiding"
+            [value]="vertrouwelijkheidaanduiding()"
+            (input)="vertrouwelijkheidaanduiding.set($any($event.target).value)"
+          />
+        </p>
+        <button utrecht-button appearance="primary-action-button" type="submit" [disabled]="saving()">
+          Opslaan
+        </button>
+      </form>
+
+      @if (saved()) {
+        <p utrecht-paragraph role="status">De standaardwaarden zijn opgeslagen.</p>
+      }
+      @if (failed()) {
+        <p utrecht-paragraph role="alert">
+          Opslaan is niet gelukt. Controleer of je als beheerder bent ingelogd en probeer het opnieuw.
+        </p>
+      }
+    } @else if (failed()) {
+      <p utrecht-paragraph role="alert">
+        Kon de standaardwaarden niet laden. Controleer of je als beheerder bent ingelogd en probeer
+        het opnieuw.
+      </p>
+    }
+  </utrecht-article>
+</main>
diff --git a/apps/beheer/src/app/default-fill/default-fill-page.spec.ts b/apps/beheer/src/app/default-fill/default-fill-page.spec.ts
new file mode 100644
index 0000000..53011c5
--- /dev/null
+++ b/apps/beheer/src/app/default-fill/default-fill-page.spec.ts
@@ -0,0 +1,90 @@
+import { signal } from '@angular/core';
+import { fireEvent, render, screen } from '@testing-library/angular';
+import { of, throwError } from 'rxjs';
+import { BeheerDefaultFill, BffApiV1Service } from 'api-client';
+import { AuthService } from 'auth';
+import { axe } from 'vitest-axe';
+import { DefaultFillPage } from './default-fill-page';
+
+const current: BeheerDefaultFill = {
+  bronorganisatie: '517439943',
+  verantwoordelijkeOrganisatie: '517439943',
+  vertrouwelijkheidaanduiding: 'openbaar',
+};
+
+class FakeAuth extends AuthService {
+  readonly isAuthenticated = signal(true);
+  readonly bsn = signal<string | undefined>(undefined);
+  override readonly roles = signal<readonly string[]>(['beheerder']);
+  login(): void {
+    /* not exercised */
+  }
+  logout(): void {
+    /* not exercised */
+  }
+}
+
+function setup(
+  overrides: {
+    getBeheerDefaultFill?: ReturnType<typeof vi.fn>;
+    putBeheerDefaultFill?: ReturnType<typeof vi.fn>;
+  } = {},
+) {
+  const getBeheerDefaultFill = overrides.getBeheerDefaultFill ?? vi.fn().mockReturnValue(of(current));
+  const putBeheerDefaultFill = overrides.putBeheerDefaultFill ?? vi.fn().mockReturnValue(of(undefined));
+  return {
+    getBeheerDefaultFill,
+    putBeheerDefaultFill,
+    providers: [
+      { provide: BffApiV1Service, useValue: { getBeheerDefaultFill, putBeheerDefaultFill } },
+      { provide: AuthService, useClass: FakeAuth },
+    ],
+  };
+}
+
+describe('DefaultFillPage', () => {
+  it('loads the current default-fill into the form on open', async () => {
+    const { getBeheerDefaultFill, providers } = setup();
+    await render(DefaultFillPage, { providers });
+
+    expect(getBeheerDefaultFill).toHaveBeenCalled();
+    const bron = (await screen.findByLabelText('Bronorganisatie')) as HTMLInputElement;
+    expect(bron.value).toBe('517439943');
+  });
+
+  it('saves the edited values via the BFF', async () => {
+    const { putBeheerDefaultFill, providers } = setup();
+    await render(DefaultFillPage, { providers });
+
+    const bron = (await screen.findByLabelText('Bronorganisatie')) as HTMLInputElement;
+    fireEvent.input(bron, { target: { value: '999999999' } });
+    fireEvent.click(screen.getByRole('button', { name: /opslaan/i }));
+
+    expect(putBeheerDefaultFill).toHaveBeenCalledWith(
+      expect.objectContaining({ bronorganisatie: '999999999', vertrouwelijkheidaanduiding: 'openbaar' }),
+    );
+    expect(await screen.findByText(/standaardwaarden zijn opgeslagen/i)).toBeTruthy();
+  });
+
+  it('surfaces a save failure instead of swallowing it', async () => {
+    const { providers } = setup({
+      putBeheerDefaultFill: vi.fn().mockReturnValue(throwError(() => new Error('403'))),
+    });
+    await render(DefaultFillPage, { providers });
+
+    fireEvent.click(await screen.findByRole('button', { name: /opslaan/i }));
+
+    expect(await screen.findByText(/opslaan is niet gelukt/i)).toBeTruthy();
+  });
+
+  it('has no WCAG 2.1 AA violations', async () => {
+    document.documentElement.lang = 'nl';
+    const { container } = await render(DefaultFillPage, { providers: setup().providers });
+
+    const results = await axe(container, {
+      runOnly: { type: 'tag', values: ['wcag2a', 'wcag2aa', 'wcag21a', 'wcag21aa'] },
+    });
+
+    expect(results.violations).toEqual([]);
+  });
+});
diff --git a/apps/beheer/src/app/default-fill/default-fill-page.ts b/apps/beheer/src/app/default-fill/default-fill-page.ts
new file mode 100644
index 0000000..7cb58e6
--- /dev/null
+++ b/apps/beheer/src/app/default-fill/default-fill-page.ts
@@ -0,0 +1,72 @@
+import { Component, inject, signal } from '@angular/core';
+import { BeheerDefaultFill, BffApiV1Service } from 'api-client';
+import { UtrechtComponentsModule } from 'ui';
+
+/**
+ * The beheer default-fill editor (S-15b): a beheerder reads and edits the ZGW default-fill values the
+ * ACL stamps on every zaak (ADR-0003). Load and save go through the BFF (`/beheer/default-fill`),
+ * which proxies the ACL (ADR-0025). A save takes effect on the next zaak (the ACL reads it per zaak).
+ */
+@Component({
+  selector: 'app-default-fill-page',
+  imports: [UtrechtComponentsModule],
+  templateUrl: './default-fill-page.html',
+})
+export class DefaultFillPage {
+  private readonly bff = inject(BffApiV1Service);
+
+  protected readonly bronorganisatie = signal('');
+  protected readonly verantwoordelijkeOrganisatie = signal('');
+  protected readonly vertrouwelijkheidaanduiding = signal('');
+  protected readonly loading = signal(false);
+  protected readonly loaded = signal(false);
+  protected readonly saving = signal(false);
+  protected readonly failed = signal(false);
+  protected readonly saved = signal(false);
+
+  constructor() {
+    this.load();
+  }
+
+  load(): void {
+    this.loading.set(true);
+    this.failed.set(false);
+    this.saved.set(false);
+    this.bff.getBeheerDefaultFill().subscribe({
+      next: (d: BeheerDefaultFill) => {
+        this.bronorganisatie.set(d.bronorganisatie);
+        this.verantwoordelijkeOrganisatie.set(d.verantwoordelijkeOrganisatie);
+        this.vertrouwelijkheidaanduiding.set(d.vertrouwelijkheidaanduiding);
+        this.loading.set(false);
+        this.loaded.set(true);
+      },
+      error: () => {
+        this.loading.set(false);
+        this.loaded.set(true);
+        this.failed.set(true);
+      },
+    });
+  }
+
+  save(): void {
+    this.saving.set(true);
+    this.failed.set(false);
+    this.saved.set(false);
+    this.bff
+      .putBeheerDefaultFill({
+        bronorganisatie: this.bronorganisatie(),
+        verantwoordelijkeOrganisatie: this.verantwoordelijkeOrganisatie(),
+        vertrouwelijkheidaanduiding: this.vertrouwelijkheidaanduiding(),
+      })
+      .subscribe({
+        next: () => {
+          this.saving.set(false);
+          this.saved.set(true);
+        },
+        error: () => {
+          this.saving.set(false);
+          this.failed.set(true);
+        },
+      });
+  }
+}
diff --git a/docs/architecture/adr-0026-mutable-default-fill-store.md b/docs/architecture/adr-0026-mutable-default-fill-store.md
new file mode 100644
index 0000000..f263298
--- /dev/null
+++ b/docs/architecture/adr-0026-mutable-default-fill-store.md
@@ -0,0 +1,61 @@
+# ADR-0026: Runtime-mutable ACL default-fill (in-memory store, seeded from config)
+
+- **Status:** Accepted
+- **Date:** 2026-07-24
+- **Deciders:** Respellion engineering
+- **Slice:** S-15b (#131), second of the S-15 (#16) split
+
+## Context
+
+ADR-0003 made the ACL *default-fill* the ZGW-mandatory fields it stamps on every
+zaak, supplied as static configuration (`Acl:Defaults`, read once at startup as an
+immutable singleton). S-15b lets a beheerder **edit** those values from the portal
+and have the next zaak reflect them — so the defaults must become mutable at runtime.
+
+Two questions: **what** is editable, and **where** the mutable state lives.
+
+## Decision
+
+**Make the three ZGW default-fill fields a runtime-mutable, in-memory store
+(`IDefaultFillStore`), seeded from `Acl:Defaults` at startup. The ACL reads it per
+zaak; the beheer `PUT /default-fill` replaces it.**
+
+### Only the three ZGW fill fields are editable
+
+`Acl:Defaults` also carries the S-27 catalog-resolution keys (`ZaaktypeIdentificatie`,
+`InformatieobjecttypeOmschrijving`). Those feed the resolved-URL cache
+(`CachedZaaktypeCatalog`, ADR-0021); editing them at runtime would leave a stale cache
+and is catalogus *wiring*, not "default fill". So they **stay static config** and are
+out of scope for the CRUD. The editable set is exactly `Bronorganisatie`,
+`VerantwoordelijkeOrganisatie`, `Vertrouwelijkheidaanduiding` (`DefaultFillSettings`).
+
+### In-memory, not persisted
+
+The store is a thread-safe in-memory singleton. **An edit is lost on restart**, when it
+reverts to the configured env. That is acceptable for this reference app: the slice
+demonstrates the *pattern* (beheer edits config that the ACL honours), not durable
+config management. The ACL stays stateless — no DB, no EF, no migration, no extra
+compose service.
+
+- ponytail ceiling: no persistence, no audit trail, no optimistic concurrency.
+- Upgrade path: back `IDefaultFillStore` with a DB (or an Objecten record) if durable,
+  audited, multi-instance config is needed — the port stays the same.
+
+## Consequences
+
+**Positive**
+
+- Demoable end to end (edit in portal → next zaak reflects it) with minimal moving parts.
+- The read path is per-zaak, so no restart and no cache concerns for the ZGW fields.
+
+**Negative / costs**
+
+- Edits don't survive a restart and aren't shared across replicas (single-instance
+  assumption). Documented ceiling above.
+- Two sources of default config now (static keys on `AclDefaults`, mutable fields in the
+  store) — a deliberate split by editability.
+
+## Coupling rules touched (CLAUDE.md §8)
+
+None new. The BFF→ACL edge already exists (ADR-0025); this adds a read/write pair on it.
+The ACL remains the owner of the ZGW-facing config.
diff --git a/docs/demo-script.md b/docs/demo-script.md
index 58b8770..2fb41c8 100644
--- a/docs/demo-script.md
+++ b/docs/demo-script.md
@@ -5,6 +5,28 @@ copy-pasteable walkthrough against a local `make up` stack.
 
 ---
 
+## S-15b — Beheer-portal: default-fill configuration editor (#131, ADR-0026)
+
+**Outcome:** a beheerder edits the ACL's ZGW **default-fill** values (bronorganisatie,
+verantwoordelijke organisatie, vertrouwelijkheidaanduiding) from the beheer portal, and the next zaak
+is stamped with the new values — no restart. Path: portal → BFF `GET/PUT /beheer/default-fill`
+(beheerder role) → ACL `GET/PUT /default-fill` → a runtime-mutable in-memory store the ACL reads per
+zaak (ADR-0026). The S-27 catalog-resolution keys stay static config (editing them would desync the
+zaaktype cache). Store is in-memory: an edit reverts to the configured env on restart.
+
+```bash
+make up
+# 1. Log in as bram-beheerder / test123 → "Default-fill" tab → change a value → Opslaan.
+open http://localhost:8143/default-fill
+#
+# 2. Automated: the ACL uses the current default-fill per zaak (unit) and the endpoints are behind the
+#    beheerder role (BFF unit):
+#    Acl.Tests → AclServiceTests.Opening_a_zaak_reflects_a_default_fill_update
+#    Bff.Tests → BeheerDefaultFillEndpointTests
+```
+
+---
+
 ## S-15a — Beheer-portal: read-only catalogus viewer (#130, ADR-0025)
 
 **Outcome:** a new **beheer** portal (medewerker realm, like behandel) shows the ZTC catalogus —
diff --git a/libs/api-client/src/lib/generated/bff-api.ts b/libs/api-client/src/lib/generated/bff-api.ts
index 1828d6e..e806978 100644
--- a/libs/api-client/src/lib/generated/bff-api.ts
+++ b/libs/api-client/src/lib/generated/bff-api.ts
@@ -24,6 +24,12 @@ import {
   Observable
 } from 'rxjs';
 
+export interface BeheerDefaultFill {
+  bronorganisatie: string;
+  verantwoordelijkeOrganisatie: string;
+  vertrouwelijkheidaanduiding: string;
+}
+
 export interface BeheerZaaktype {
   identificatie: string;
   omschrijving: string;
@@ -446,4 +452,69 @@ export class BffApiV1Service {
     );
   }
 
+ getBeheerDefaultFill<TData = BeheerDefaultFill>( options?: HttpClientBodyOptions): Observable<TData>;
+ getBeheerDefaultFill<TData = BeheerDefaultFill>( options?: HttpClientEventOptions): Observable<HttpEvent<TData>>;
+ getBeheerDefaultFill<TData = BeheerDefaultFill>( options?: HttpClientResponseOptions): Observable<AngularHttpResponse<TData>>;
+  getBeheerDefaultFill<TData = BeheerDefaultFill>(
+     options?: HttpClientObserveOptions): Observable<TData | HttpEvent<TData> | AngularHttpResponse<TData>> {
+    if (options?.observe === 'events') {
+      return this.http.get<TData>(
+      `/beheer/default-fill`,{
+        ...(options as Omit<NonNullable<typeof options>, 'observe'>),
+        observe: 'events',
+      }
+    );
+    }
+
+    if (options?.observe === 'response') {
+      return this.http.get<TData>(
+      `/beheer/default-fill`,{
+        ...(options as Omit<NonNullable<typeof options>, 'observe'>),
+        observe: 'response',
+      }
+    );
+    }
+
+    return this.http.get<TData>(
+      `/beheer/default-fill`,{
+        ...(options as Omit<NonNullable<typeof options>, 'observe'>),
+        observe: 'body',
+      }
+    );
+  }
+
+ putBeheerDefaultFill<TData = void>(beheerDefaultFill: BeheerDefaultFill, options?: HttpClientBodyOptions): Observable<TData>;
+ putBeheerDefaultFill<TData = void>(beheerDefaultFill: BeheerDefaultFill, options?: HttpClientEventOptions): Observable<HttpEvent<TData>>;
+ putBeheerDefaultFill<TData = void>(beheerDefaultFill: BeheerDefaultFill, options?: HttpClientResponseOptions): Observable<AngularHttpResponse<TData>>;
+  putBeheerDefaultFill<TData = void>(
+    beheerDefaultFill: BeheerDefaultFill, options?: HttpClientObserveOptions): Observable<TData | HttpEvent<TData> | AngularHttpResponse<TData>> {
+    if (options?.observe === 'events') {
+      return this.http.put<TData>(
+      `/beheer/default-fill`,
+      beheerDefaultFill,{
+        ...(options as Omit<NonNullable<typeof options>, 'observe'>),
+        observe: 'events',
+      }
+    );
+    }
+
+    if (options?.observe === 'response') {
+      return this.http.put<TData>(
+      `/beheer/default-fill`,
+      beheerDefaultFill,{
+        ...(options as Omit<NonNullable<typeof options>, 'observe'>),
+        observe: 'response',
+      }
+    );
+    }
+
+    return this.http.put<TData>(
+      `/beheer/default-fill`,
+      beheerDefaultFill,{
+        ...(options as Omit<NonNullable<typeof options>, 'observe'>),
+        observe: 'body',
+      }
+    );
+  }
+
 };
diff --git a/services/acl/Acl.Api/Program.cs b/services/acl/Acl.Api/Program.cs
index 21509c8..9b1b7bc 100644
--- a/services/acl/Acl.Api/Program.cs
+++ b/services/acl/Acl.Api/Program.cs
@@ -33,6 +33,15 @@ builder.Services.AddSingleton(sp => sp.GetRequiredService<IConfiguration>()
 builder.Services.AddSingleton(sp => sp.GetRequiredService<IConfiguration>()
     .GetSection("Acl:OpenZaak").Get<OpenZaakOptions>()
     ?? throw new InvalidOperationException("Missing configuration section 'Acl:OpenZaak'"));
+// The default-fill values are held in a runtime-mutable store (S-15b, ADR-0026), seeded from the
+// configured Acl:Defaults. The beheer portal edits it; the worker reads it per zaak. The S-27
+// resolution keys stay on AclDefaults (static) — see DefaultFillSettings.
+builder.Services.AddSingleton<IDefaultFillStore>(sp =>
+{
+    var d = sp.GetRequiredService<AclDefaults>();
+    return new InMemoryDefaultFillStore(
+        new DefaultFillSettings(d.Bronorganisatie, d.VerantwoordelijkeOrganisatie, d.Vertrouwelijkheidaanduiding));
+});
 builder.Services.AddHttpClient<IZaakGateway, OpenZaakGateway>();
 // Singleton so the resolved zaaktype/informatieobjecttype URLs are cached across requests (S-27).
 builder.Services.AddSingleton<IZaaktypeCatalog, CachedZaaktypeCatalog>();
@@ -91,6 +100,22 @@ app.MapPost("/documenten", async (StoreDocumentRequest body, AclService acl, Can
 app.MapGet("/catalogi/zaaktypen", async (AclService acl, CancellationToken ct) =>
     Results.Ok(await acl.ListZaaktypenAsync(ct)));
 
+// Read the current default-fill settings (beheer config viewer, S-15b).
+app.MapGet("/default-fill", (AclService acl) => Results.Ok(acl.GetDefaultFill()));
+
+// Update the default-fill settings from the beheer portal (S-15b). Behind beheerder authorization at
+// the BFF; the ACL validates the values are present (the three ZGW-mandatory fields).
+app.MapPut("/default-fill", (DefaultFillSettings body, AclService acl) =>
+{
+    if (string.IsNullOrWhiteSpace(body.Bronorganisatie) ||
+        string.IsNullOrWhiteSpace(body.VerantwoordelijkeOrganisatie) ||
+        string.IsNullOrWhiteSpace(body.Vertrouwelijkheidaanduiding))
+        return Results.BadRequest(new { error = "bronorganisatie, verantwoordelijkeOrganisatie and vertrouwelijkheidaanduiding are all required." });
+
+    acl.UpdateDefaultFill(body);
+    return Results.NoContent();
+});
+
 app.Run();
 
 public sealed record OpenZaakRequest(string Bsn, string Reference);
diff --git a/services/acl/Acl.Application/AclService.cs b/services/acl/Acl.Application/AclService.cs
index 52bb9d7..28e83f9 100644
--- a/services/acl/Acl.Application/AclService.cs
+++ b/services/acl/Acl.Application/AclService.cs
@@ -2,12 +2,15 @@ namespace Acl.Application;
 
 /// <summary>The ACL's single operation: open a zaak from a domain payload,
 /// default-filling the ZGW-mandatory fields (ADR-0003).</summary>
-public sealed class AclService(IZaakGateway gateway, AclDefaults defaults, IZaaktypeCatalog catalog, IClock clock)
+public sealed class AclService(IZaakGateway gateway, IDefaultFillStore fill, IZaaktypeCatalog catalog, IClock clock)
 {
     public async Task<Uri> OpenZaakAsync(DomainRegistration registration, CancellationToken ct = default)
     {
         ArgumentNullException.ThrowIfNull(registration);
 
+        // Read the current default-fill per zaak (not at construction), so a beheerder edit (S-15b)
+        // takes effect on the next zaak without a restart.
+        var defaults = fill.Current;
         var request = new ZaakRequest(
             defaults.Bronorganisatie,
             defaults.VerantwoordelijkeOrganisatie,
@@ -47,6 +50,17 @@ public sealed class AclService(IZaakGateway gateway, AclDefaults defaults, IZaak
     public Task<IReadOnlyList<ZaaktypeSummary>> ListZaaktypenAsync(CancellationToken ct = default) =>
         gateway.ListZaaktypenAsync(ct);
 
+    /// <summary>The current default-fill settings, for the beheer config viewer (S-15b).</summary>
+    public DefaultFillSettings GetDefaultFill() => fill.Current;
+
+    /// <summary>Replace the default-fill settings from the beheer portal (S-15b). Takes effect on the
+    /// next zaak (the fill is read per zaak, not cached).</summary>
+    public void UpdateDefaultFill(DefaultFillSettings settings)
+    {
+        ArgumentNullException.ThrowIfNull(settings);
+        fill.Update(settings);
+    }
+
     /// <summary>The zaak's reference (its ZGW identificatie), for the read projection (#78).</summary>
     public Task<string> GetZaakReferenceAsync(Uri zaakUrl, CancellationToken ct = default)
     {
@@ -68,6 +82,7 @@ public sealed class AclService(IZaakGateway gateway, AclDefaults defaults, IZaak
         ArgumentException.ThrowIfNullOrWhiteSpace(fileName);
         ArgumentException.ThrowIfNullOrWhiteSpace(contentType);
 
+        var defaults = fill.Current;
         var request = new DocumentRequest(
             defaults.Bronorganisatie,
             await catalog.GetInformatieobjecttypeUrlAsync(ct),
diff --git a/services/acl/Acl.Application/DefaultFillSettings.cs b/services/acl/Acl.Application/DefaultFillSettings.cs
new file mode 100644
index 0000000..2978c4e
--- /dev/null
+++ b/services/acl/Acl.Application/DefaultFillSettings.cs
@@ -0,0 +1,10 @@
+namespace Acl.Application;
+
+/// <summary>The ZGW default-fill values a beheerder can edit at runtime (S-15b) — the mandatory fields
+/// the ACL stamps on every zaak (ADR-0003). The S-27 catalog-resolution keys (zaaktype identificatie,
+/// informatieobjecttype omschrijving) stay static config: editing them would desync the resolved-URL
+/// cache, and they're catalogus wiring rather than "default fill".</summary>
+public sealed record DefaultFillSettings(
+    string Bronorganisatie,
+    string VerantwoordelijkeOrganisatie,
+    string Vertrouwelijkheidaanduiding);
diff --git a/services/acl/Acl.Application/IDefaultFillStore.cs b/services/acl/Acl.Application/IDefaultFillStore.cs
new file mode 100644
index 0000000..d434dec
--- /dev/null
+++ b/services/acl/Acl.Application/IDefaultFillStore.cs
@@ -0,0 +1,14 @@
+namespace Acl.Application;
+
+/// <summary>Holds the ACL's current default-fill values, editable at runtime through the beheer portal
+/// (S-15b). Seeded from config at startup.
+///
+/// ponytail: in-memory only — an edit is lost on restart, when it reverts to the configured env
+/// (ADR-0026). Adequate for the reference demo; back it with a DB if durable, audited config is needed.
+/// </summary>
+public interface IDefaultFillStore
+{
+    DefaultFillSettings Current { get; }
+
+    void Update(DefaultFillSettings settings);
+}
diff --git a/services/acl/Acl.Application/InMemoryDefaultFillStore.cs b/services/acl/Acl.Application/InMemoryDefaultFillStore.cs
new file mode 100644
index 0000000..b6f5d5a
--- /dev/null
+++ b/services/acl/Acl.Application/InMemoryDefaultFillStore.cs
@@ -0,0 +1,20 @@
+namespace Acl.Application;
+
+/// <summary>In-memory <see cref="IDefaultFillStore"/> (ADR-0026), seeded from config. Thread-safe: the
+/// hosted worker reads <see cref="Current"/> per zaak while the beheer endpoint may update it.</summary>
+public sealed class InMemoryDefaultFillStore(DefaultFillSettings seed) : IDefaultFillStore
+{
+    private readonly object _gate = new();
+    private DefaultFillSettings _current = seed;
+
+    public DefaultFillSettings Current
+    {
+        get { lock (_gate) return _current; }
+    }
+
+    public void Update(DefaultFillSettings settings)
+    {
+        ArgumentNullException.ThrowIfNull(settings);
+        lock (_gate) _current = settings;
+    }
+}
diff --git a/services/acl/Acl.Tests/AclServiceTests.cs b/services/acl/Acl.Tests/AclServiceTests.cs
index 905dc9f..6b726f8 100644
--- a/services/acl/Acl.Tests/AclServiceTests.cs
+++ b/services/acl/Acl.Tests/AclServiceTests.cs
@@ -84,8 +84,11 @@ public class AclServiceTests
         InformatieobjecttypeOmschrijving = "Diploma",
     };
 
+    private static InMemoryDefaultFillStore FillFrom(AclDefaults d) =>
+        new(new DefaultFillSettings(d.Bronorganisatie, d.VerantwoordelijkeOrganisatie, d.Vertrouwelijkheidaanduiding));
+
     private static AclService ServiceWith(FakeGateway gateway, AclDefaults defaults, DateOnly today) =>
-        new(gateway, defaults, new CachedZaaktypeCatalog(gateway, defaults), new FixedClock(today));
+        new(gateway, FillFrom(defaults), new CachedZaaktypeCatalog(gateway, defaults), new FixedClock(today));
 
     private sealed class FixedClock(DateOnly today) : IClock
     {
@@ -113,6 +116,22 @@ public class AclServiceTests
         Assert.Equal("reg-77", req.Identificatie);
     }
 
+    [Fact]
+    public async Task Opening_a_zaak_reflects_a_default_fill_update(/* S-15b */)
+    {
+        var gateway = new FakeGateway();
+        var service = ServiceWith(gateway, Defaults(), new DateOnly(2026, 6, 4));
+
+        // A beheerder edits the default-fill; the very next zaak must use the new values (read per zaak).
+        service.UpdateDefaultFill(new DefaultFillSettings("999999999", "888888888", "vertrouwelijk"));
+        await service.OpenZaakAsync(new DomainRegistration("123456782", "reg-1"));
+
+        var req = gateway.Captured!;
+        Assert.Equal("999999999", req.Bronorganisatie);
+        Assert.Equal("888888888", req.VerantwoordelijkeOrganisatie);
+        Assert.Equal("vertrouwelijk", req.Vertrouwelijkheidaanduiding);
+    }
+
     [Fact]
     public async Task Rejects_a_null_registration_without_calling_the_gateway()
     {
diff --git a/services/acl/Acl.Tests/DefaultFillStoreTests.cs b/services/acl/Acl.Tests/DefaultFillStoreTests.cs
new file mode 100644
index 0000000..596aefc
--- /dev/null
+++ b/services/acl/Acl.Tests/DefaultFillStoreTests.cs
@@ -0,0 +1,29 @@
+using Acl.Application;
+
+namespace Acl.Tests;
+
+public class DefaultFillStoreTests
+{
+    private static DefaultFillSettings Seed() => new("517439943", "517439943", "openbaar");
+
+    [Fact]
+    public void Seeds_from_the_supplied_settings()
+    {
+        var store = new InMemoryDefaultFillStore(Seed());
+
+        Assert.Equal("517439943", store.Current.Bronorganisatie);
+        Assert.Equal("openbaar", store.Current.Vertrouwelijkheidaanduiding);
+    }
+
+    [Fact]
+    public void Updating_replaces_the_current_settings()
+    {
+        var store = new InMemoryDefaultFillStore(Seed());
+
+        store.Update(new DefaultFillSettings("999999999", "888888888", "vertrouwelijk"));
+
+        Assert.Equal("999999999", store.Current.Bronorganisatie);
+        Assert.Equal("888888888", store.Current.VerantwoordelijkeOrganisatie);
+        Assert.Equal("vertrouwelijk", store.Current.Vertrouwelijkheidaanduiding);
+    }
+}
diff --git a/services/bff/Bff.Api/DownstreamClients.cs b/services/bff/Bff.Api/DownstreamClients.cs
index f9761d5..8696b66 100644
--- a/services/bff/Bff.Api/DownstreamClients.cs
+++ b/services/bff/Bff.Api/DownstreamClients.cs
@@ -59,12 +59,26 @@ public interface IProjectionClient
 /// internal reference, not shown in the portal.</summary>
 public sealed record BeheerZaaktype(string Identificatie, string Omschrijving);
 
-/// <summary>Port to the ACL for read-only catalogus queries (beheer portal, S-15a). The BFF reaches the
-/// ACL directly for this read: the catalogus isn't a domain concern, and the ACL is the only code
-/// allowed to read the ZGW Catalogi API (§8.1, ADR-0025).</summary>
+/// <summary>The ACL default-fill settings the beheer portal reads + edits (S-15b): the three ZGW-mandatory
+/// fields the ACL stamps on every zaak (ADR-0003).</summary>
+public sealed record BeheerDefaultFill(
+    string Bronorganisatie,
+    string VerantwoordelijkeOrganisatie,
+    string Vertrouwelijkheidaanduiding);
+
+/// <summary>Port to the ACL for beheer queries (beheer portal). The BFF reaches the ACL directly: these
+/// aren't a domain concern, and the ACL is the only code allowed to read/own the ZGW-facing config
+/// (§8.1, ADR-0025).</summary>
 public interface IAclClient
 {
+    /// <summary>The published catalogus zaaktypen, read-only (S-15a).</summary>
     Task<IReadOnlyList<BeheerZaaktype>> GetZaaktypenAsync(CancellationToken ct = default);
+
+    /// <summary>The current default-fill settings (S-15b).</summary>
+    Task<BeheerDefaultFill> GetDefaultFillAsync(CancellationToken ct = default);
+
+    /// <summary>Replace the default-fill settings (S-15b).</summary>
+    Task UpdateDefaultFillAsync(BeheerDefaultFill settings, CancellationToken ct = default);
 }
 
 /// <summary>Calls the Domain Service's <c>POST /registrations</c>.</summary>
@@ -141,4 +155,14 @@ public sealed class AclClient(HttpClient http) : IAclClient
 {
     public async Task<IReadOnlyList<BeheerZaaktype>> GetZaaktypenAsync(CancellationToken ct = default)
         => await http.GetFromJsonAsync<List<BeheerZaaktype>>("catalogi/zaaktypen", ct) ?? [];
+
+    public async Task<BeheerDefaultFill> GetDefaultFillAsync(CancellationToken ct = default)
+        => await http.GetFromJsonAsync<BeheerDefaultFill>("default-fill", ct)
+           ?? throw new InvalidOperationException("The ACL returned an empty default-fill response.");
+
+    public async Task UpdateDefaultFillAsync(BeheerDefaultFill settings, CancellationToken ct = default)
+    {
+        using var response = await http.PutAsJsonAsync("default-fill", settings, ct);
+        response.EnsureSuccessStatusCode();
+    }
 }
diff --git a/services/bff/Bff.Api/Program.cs b/services/bff/Bff.Api/Program.cs
index dd9a8f2..ee2a36e 100644
--- a/services/bff/Bff.Api/Program.cs
+++ b/services/bff/Bff.Api/Program.cs
@@ -228,6 +228,25 @@ app.MapGet("/beheer/catalogi/zaaktypen", async (IAclClient acl, CancellationToke
     .Produces(StatusCodes.Status401Unauthorized)
     .Produces(StatusCodes.Status403Forbidden);
 
+// Beheer default-fill config (S-15b): read + edit the ACL's default-fill values. Behind medewerker-
+// realm + beheerder authorization; the BFF proxies the ACL (ADR-0025). The ACL validates the values.
+app.MapGet("/beheer/default-fill", async (IAclClient acl, CancellationToken ct) =>
+    Results.Ok(await acl.GetDefaultFillAsync(ct)))
+    .RequireAuthorization(BeheerAuth.Policy)
+    .Produces<BeheerDefaultFill>(StatusCodes.Status200OK)
+    .Produces(StatusCodes.Status401Unauthorized)
+    .Produces(StatusCodes.Status403Forbidden);
+
+app.MapPut("/beheer/default-fill", async (BeheerDefaultFill body, IAclClient acl, CancellationToken ct) =>
+{
+    await acl.UpdateDefaultFillAsync(body, ct);
+    return Results.NoContent();
+})
+    .RequireAuthorization(BeheerAuth.Policy)
+    .Produces(StatusCodes.Status204NoContent)
+    .Produces(StatusCodes.Status401Unauthorized)
+    .Produces(StatusCodes.Status403Forbidden);
+
 app.Run();
 
 /// <summary>The behandelaar's decision on a registration.</summary>
diff --git a/services/bff/Bff.Tests/BeheerDefaultFillEndpointTests.cs b/services/bff/Bff.Tests/BeheerDefaultFillEndpointTests.cs
new file mode 100644
index 0000000..472c664
--- /dev/null
+++ b/services/bff/Bff.Tests/BeheerDefaultFillEndpointTests.cs
@@ -0,0 +1,84 @@
+using System.Net;
+using System.Net.Http.Headers;
+using System.Net.Http.Json;
+using Bff.Api;
+
+namespace Bff.Tests;
+
+/// <summary>
+/// The beheer default-fill config endpoints (S-15b): read (GET) and edit (PUT) the ACL's default-fill,
+/// reached only with a medewerker-realm token carrying the <c>beheerder</c> role. Missing token → 401;
+/// a medewerker without the role → 403; a beheerder reads and updates via the ACL client.
+/// </summary>
+public class BeheerDefaultFillEndpointTests
+{
+    private static HttpRequestMessage Get(string? bearer)
+    {
+        var r = new HttpRequestMessage(HttpMethod.Get, "/beheer/default-fill");
+        if (bearer is not null) r.Headers.Authorization = new AuthenticationHeaderValue("Bearer", bearer);
+        return r;
+    }
+
+    private static HttpRequestMessage Put(string? bearer, object body)
+    {
+        var r = new HttpRequestMessage(HttpMethod.Put, "/beheer/default-fill") { Content = JsonContent.Create(body) };
+        if (bearer is not null) r.Headers.Authorization = new AuthenticationHeaderValue("Bearer", bearer);
+        return r;
+    }
+
+    [Fact]
+    public async Task Rejects_read_without_a_token()
+    {
+        using var factory = new BffFactory();
+        var response = await factory.CreateClient().SendAsync(Get(bearer: null));
+        Assert.Equal(HttpStatusCode.Unauthorized, response.StatusCode);
+    }
+
+    [Fact]
+    public async Task Rejects_a_medewerker_without_the_beheerder_role()
+    {
+        using var factory = new BffFactory();
+        var response = await factory.CreateClient().SendAsync(Get(TestTokens.Medewerker("behandelaar")));
+        Assert.Equal(HttpStatusCode.Forbidden, response.StatusCode);
+    }
+
+    [Fact]
+    public async Task Serves_the_current_default_fill_to_a_beheerder()
+    {
+        using var factory = new BffFactory();
+        factory.Acl.DefaultFill = new BeheerDefaultFill("517439943", "517439943", "openbaar");
+
+        var response = await factory.CreateClient().SendAsync(Get(TestTokens.Medewerker("beheerder")));
+
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+        var body = await response.Content.ReadFromJsonAsync<BeheerDefaultFill>();
+        Assert.Equal("517439943", body!.Bronorganisatie);
+        Assert.Equal("openbaar", body.Vertrouwelijkheidaanduiding);
+    }
+
+    [Fact]
+    public async Task Updates_the_default_fill_via_the_acl_for_a_beheerder()
+    {
+        using var factory = new BffFactory();
+
+        var response = await factory.CreateClient().SendAsync(
+            Put(TestTokens.Medewerker("beheerder"),
+                new { bronorganisatie = "999999999", verantwoordelijkeOrganisatie = "888888888", vertrouwelijkheidaanduiding = "vertrouwelijk" }));
+
+        Assert.Equal(HttpStatusCode.NoContent, response.StatusCode);
+        Assert.Equal("999999999", factory.Acl.Updated!.Bronorganisatie);
+        Assert.Equal("vertrouwelijk", factory.Acl.Updated.Vertrouwelijkheidaanduiding);
+    }
+
+    [Fact]
+    public async Task Rejects_an_update_from_a_non_beheerder()
+    {
+        using var factory = new BffFactory();
+
+        var response = await factory.CreateClient().SendAsync(
+            Put(TestTokens.Medewerker("behandelaar"), new { bronorganisatie = "1", verantwoordelijkeOrganisatie = "2", vertrouwelijkheidaanduiding = "openbaar" }));
+
+        Assert.Equal(HttpStatusCode.Forbidden, response.StatusCode);
+        Assert.Null(factory.Acl.Updated);
+    }
+}
diff --git a/services/bff/Bff.Tests/BffFactory.cs b/services/bff/Bff.Tests/BffFactory.cs
index 7f1141e..278a61c 100644
--- a/services/bff/Bff.Tests/BffFactory.cs
+++ b/services/bff/Bff.Tests/BffFactory.cs
@@ -142,11 +142,24 @@ internal sealed class FakeProjectionClient : IProjectionClient
         => Task.FromResult<IReadOnlyList<ProjectionEntry>>(Entries);
 }
 
-/// <summary>Serves a configurable set of catalogus zaaktypen (beheer viewer, S-15a).</summary>
+/// <summary>Serves catalogus zaaktypen (S-15a) and holds the default-fill settings (S-15b).</summary>
 internal sealed class FakeAclClient : IAclClient
 {
     public List<BeheerZaaktype> Zaaktypen { get; } = [];
 
     public Task<IReadOnlyList<BeheerZaaktype>> GetZaaktypenAsync(CancellationToken ct = default)
         => Task.FromResult<IReadOnlyList<BeheerZaaktype>>(Zaaktypen);
+
+    public BeheerDefaultFill DefaultFill { get; set; } = new("517439943", "517439943", "openbaar");
+    public BeheerDefaultFill? Updated { get; private set; }
+
+    public Task<BeheerDefaultFill> GetDefaultFillAsync(CancellationToken ct = default)
+        => Task.FromResult(DefaultFill);
+
+    public Task UpdateDefaultFillAsync(BeheerDefaultFill settings, CancellationToken ct = default)
+    {
+        Updated = settings;
+        DefaultFill = settings;
+        return Task.CompletedTask;
+    }
 }
diff --git a/services/bff/openapi.json b/services/bff/openapi.json
index 10b790d..a9a985c 100644
--- a/services/bff/openapi.json
+++ b/services/bff/openapi.json
@@ -255,10 +255,80 @@
           }
         }
       }
+    },
+    "/beheer/default-fill": {
+      "get": {
+        "tags": [
+          "Bff.Api"
+        ],
+        "responses": {
+          "200": {
+            "description": "OK",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/BeheerDefaultFill"
+                }
+              }
+            }
+          },
+          "401": {
+            "description": "Unauthorized"
+          },
+          "403": {
+            "description": "Forbidden"
+          }
+        }
+      },
+      "put": {
+        "tags": [
+          "Bff.Api"
+        ],
+        "requestBody": {
+          "content": {
+            "application/json": {
+              "schema": {
+                "$ref": "#/components/schemas/BeheerDefaultFill"
+              }
+            }
+          },
+          "required": true
+        },
+        "responses": {
+          "204": {
+            "description": "No Content"
+          },
+          "401": {
+            "description": "Unauthorized"
+          },
+          "403": {
+            "description": "Forbidden"
+          }
+        }
+      }
     }
   },
   "components": {
     "schemas": {
+      "BeheerDefaultFill": {
+        "required": [
+          "bronorganisatie",
+          "verantwoordelijkeOrganisatie",
+          "vertrouwelijkheidaanduiding"
+        ],
+        "type": "object",
+        "properties": {
+          "bronorganisatie": {
+            "type": "string"
+          },
+          "verantwoordelijkeOrganisatie": {
+            "type": "string"
+          },
+          "vertrouwelijkheidaanduiding": {
+            "type": "string"
+          }
+        }
+      },
       "BeheerZaaktype": {
         "required": [
           "identificatie",
diff --git a/tests/acceptance/Steps/EenZaakOpenenSteps.cs b/tests/acceptance/Steps/EenZaakOpenenSteps.cs
index 493fb55..b65a04f 100644
--- a/tests/acceptance/Steps/EenZaakOpenenSteps.cs
+++ b/tests/acceptance/Steps/EenZaakOpenenSteps.cs
@@ -44,7 +44,9 @@ public sealed class EenZaakOpenenSteps
     [When("the domain asks the ACL to open a zaak")]
     public async Task WhenTheDomainAsksTheAclToOpenAZaak()
     {
-        var service = new AclService(_gateway, _defaults!, new CachedZaaktypeCatalog(_gateway, _defaults!), new FixedClock(_today));
+        var fill = new InMemoryDefaultFillStore(new DefaultFillSettings(
+            _defaults!.Bronorganisatie, _defaults.VerantwoordelijkeOrganisatie, _defaults.Vertrouwelijkheidaanduiding));
+        var service = new AclService(_gateway, fill, new CachedZaaktypeCatalog(_gateway, _defaults!), new FixedClock(_today));
         _returnedUrl = await service.OpenZaakAsync(_registration!);
     }
 
diff --git a/tests/e2e/default-fill.spec.ts b/tests/e2e/default-fill.spec.ts
new file mode 100644
index 0000000..8ff8d03
--- /dev/null
+++ b/tests/e2e/default-fill.spec.ts
@@ -0,0 +1,26 @@
+import { expect, test } from '@playwright/test';
+
+// S-15b: a beheerder edits the ACL default-fill in the beheer portal and gets a saved confirmation.
+// Runs against the shared verify stack; it edits + saves (the ACL store is in-memory, ADR-0026) and
+// asserts the confirmation, without depending on another test's state.
+test('a beheerder edits and saves the default-fill', async ({ page }) => {
+  await page.goto('http://beheer/');
+
+  // Keycloak medewerker-realm login (same realm as behandel).
+  await page.locator('#username').fill('bram-beheerder');
+  await page.locator('#password').fill('test123');
+  await page.locator('#kc-login').click();
+
+  await expect(page.getByRole('heading', { name: /Catalogus/i })).toBeVisible();
+
+  // Navigate to the default-fill editor and change a value.
+  await page.getByRole('link', { name: /Default-fill/i }).click();
+  await expect(page.getByRole('heading', { name: /Default-fill/i })).toBeVisible();
+
+  const bron = page.getByLabel('Bronorganisatie');
+  await expect(bron).toBeVisible();
+  await bron.fill('517439943');
+  await page.getByRole('button', { name: /Opslaan/i }).click();
+
+  await expect(page.getByText(/standaardwaarden zijn opgeslagen/i)).toBeVisible();
+});