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..de4ede6
--- /dev/null
+++ b/services/acl/Acl.Application/InMemoryDefaultFillStore.cs
@@ -0,0 +1,19 @@
+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)
+    {
+        // ponytail: red stub — real assignment lands in the green commit.
+    }
+}
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);
+    }
+}