diff --git a/.gitignore b/.gitignore
index 21f4674..91aea34 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,6 +5,9 @@ obj/
 [Rr]elease/
 *.user
 
+# Reqnroll-generated test code (regenerated from *.feature on build)
+*.feature.cs
+
 # Test results / coverage
 [Tt]est[Rr]esults/
 *.trx
diff --git a/register-referentie.slnx b/register-referentie.slnx
index bf80fac..fe5cf56 100644
--- a/register-referentie.slnx
+++ b/register-referentie.slnx
@@ -10,4 +10,7 @@
     <Project Path="services/bff/Bff.Api/Bff.Api.csproj" />
     <Project Path="services/bff/Bff.Tests/Bff.Tests.csproj" />
   </Folder>
+  <Folder Name="/tests/">
+    <Project Path="tests/acceptance/Acceptance.csproj" />
+  </Folder>
 </Solution>
diff --git a/tests/acceptance/Acceptance.csproj b/tests/acceptance/Acceptance.csproj
new file mode 100644
index 0000000..a1f3232
--- /dev/null
+++ b/tests/acceptance/Acceptance.csproj
@@ -0,0 +1,22 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" Version="6.0.4" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
+    <PackageReference Include="Reqnroll.xUnit" Version="3.3.4" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.4" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\services\acl\Acl.Application\Acl.Application.csproj" />
+    <ProjectReference Include="..\..\services\acl\Acl.Infrastructure\Acl.Infrastructure.csproj" />
+  </ItemGroup>
+
+</Project>
diff --git a/tests/acceptance/Features/EenZaakOpenen.feature b/tests/acceptance/Features/EenZaakOpenen.feature
new file mode 100644
index 0000000..8b7e41c
--- /dev/null
+++ b/tests/acceptance/Features/EenZaakOpenen.feature
@@ -0,0 +1,27 @@
+# language: en
+# Drives S-04 (#5). The ACL is the only code that talks to ZGW (ADR-0001); it
+# default-fills the ZGW-mandatory zaak fields the domain never sees (ADR-0003).
+# Real-OpenZaak verification is a separate slice — this scenario exercises the
+# use case against an in-memory stand-in for the Zaken API.
+Feature: Een zaak openen vanuit een registratie
+  Als domein wil ik de ACL vragen een zaak te openen
+  zodat de ZGW-verplichte velden worden ingevuld zonder dat het domein ze kent.
+
+  Scenario: De ACL vult de ZGW-verplichte velden default in
+    Given a domain registration for BSN "123456782"
+    And the ACL is configured with these defaults:
+      | field                        | value                                         |
+      | bronorganisatie              | 517439943                                     |
+      | verantwoordelijkeOrganisatie | 517439943                                     |
+      | vertrouwelijkheidaanduiding  | openbaar                                      |
+      | zaaktype                     | http://openzaak/catalogi/api/v1/zaaktypen/big |
+    And today is "2026-06-04"
+    When the domain asks the ACL to open a zaak
+    Then a zaak is created with these default-filled fields
+      | field                        | value                                         |
+      | bronorganisatie              | 517439943                                     |
+      | verantwoordelijkeOrganisatie | 517439943                                     |
+      | vertrouwelijkheidaanduiding  | openbaar                                      |
+      | zaaktype                     | http://openzaak/catalogi/api/v1/zaaktypen/big |
+      | startdatum                   | 2026-06-04                                    |
+    And the ACL returns the URL of the created zaak