diff --git a/.gitea/workflows/ci.yaml b/.gitea/workflows/ci.yaml
index 9e3c95b..16901c6 100644
--- a/.gitea/workflows/ci.yaml
+++ b/.gitea/workflows/ci.yaml
@@ -94,10 +94,12 @@ jobs:
         run: make verify-nrc
       - name: OpenZaak → NRC → Event Subscriber → projection-api
         run: make verify-projection
+      - name: Domain → Flowable → ACL → OpenZaak
+        run: make verify-domain
       # Log dump must precede teardown (which removes the containers).
       - name: Dump container logs on failure
         if: failure()
-        run: docker compose -f infra/docker-compose.yml logs --no-color --tail=100 oz-init openzaak nrc-init nrc-web nrc-celery nrc-beat flowable-init keycloak acl bff projection-db event-subscriber projection-api 2>&1 || true
+        run: docker compose -f infra/docker-compose.yml logs --no-color --tail=100 oz-init openzaak nrc-init nrc-web nrc-celery nrc-beat flowable-db flowable-rest flowable-init keycloak acl bff domain projection-db event-subscriber projection-api 2>&1 || true
       - name: Tear down
         if: always()
         run: make down
diff --git a/Makefile b/Makefile
index 0f65935..b6fba46 100644
--- a/Makefile
+++ b/Makefile
@@ -10,7 +10,7 @@ COMPOSE    := infra/docker-compose.yml
 # Long-running services with a healthcheck — the smoke polls these for readiness
 # (infra/wait-healthy.sh). One-shot init jobs (oz-init, nrc-init, flowable-init)
 # are not polled; they only need to have run. See docs/runbooks/gitea-actions-gotchas.md.
-WAIT_SVCS  := openzaak nrc-web acl bff event-subscriber projection-api
+WAIT_SVCS  := openzaak nrc-web acl bff domain event-subscriber projection-api
 # Config files (OpenZaak data.yaml, Keycloak realms, Flowable BPMN) are streamed
 # into external named volumes via `docker cp` (infra/seed-config.sh) instead of
 # bind-mounted, because bind mounts don't reach sibling containers on the
@@ -138,6 +138,11 @@ verify-nrc:
 verify-projection:
 	bash infra/run-projection-check.sh
 
+## verify-domain: domain → Flowable → ACL → OpenZaak end-to-end (S-05), against the
+## already-running stack. Recreates the acl service to inject the seeded zaaktype URL.
+verify-domain:
+	bash infra/run-domain-check.sh
+
 ## verify: local mirror of the CI verify-stack job — full stack up once, all checks,
 ## tear down (always). For fast single-concern local iteration use `integration`
 ## (oz-only) or `verify-notifications` (oz+nrc) instead.
@@ -148,7 +153,8 @@ verify:
 	  WAIT_TIMEOUT=420 bash infra/wait-healthy.sh $(WAIT_SVCS) \
 	    && bash infra/run-acl-integration.sh \
 	    && bash infra/run-notification-check.sh \
-	    && bash infra/run-projection-check.sh || rc=$$?; \
+	    && bash infra/run-projection-check.sh \
+	    && bash infra/run-domain-check.sh || rc=$$?; \
 	  docker compose -f $(COMPOSE) down --volumes >/dev/null 2>&1; \
 	  docker volume rm -f $(CFG_VOLS) >/dev/null 2>&1; \
 	  exit $$rc'
diff --git a/infra/docker-compose.yml b/infra/docker-compose.yml
index ecbe7bd..02a4e90 100644
--- a/infra/docker-compose.yml
+++ b/infra/docker-compose.yml
@@ -285,7 +285,9 @@ services:
       dockerfile: Dockerfile
     image: register-referentie/acl:dev
     environment:
-      Acl__OpenZaak__BaseUrl: http://openzaak:8000/
+      # Overridable so verify-domain can point the ACL at the same OpenZaak host that
+      # owns the seeded zaaktype URL (host-consistent zaak creation, ADR-0009).
+      Acl__OpenZaak__BaseUrl: ${ACL_OPENZAAK_BASEURL:-http://openzaak:8000/}
       Acl__OpenZaak__ClientId: big-reference-seed
       Acl__OpenZaak__Secret: insecure-dev-secret-change-me
       Acl__Defaults__Bronorganisatie: "517439943"
@@ -306,6 +308,36 @@ services:
         condition: service_healthy
     networks: [cg]
 
+  # ── BIG Domain Service (S-05) ──────────────────────────────────────────────
+  # Orchestrates a registration: POST /registrations creates the aggregate and
+  # starts the registratie Flowable process; a hosted worker acquires the
+  # OpenZaakAanmaken job, opens a zaak via the ACL and completes it (ADR-0009).
+  # Talks only to Flowable (Workflow Client, §8.2) and the ACL (§8.1).
+  domain:
+    build:
+      context: ../services/domain
+      dockerfile: Dockerfile
+    image: register-referentie/domain:dev
+    environment:
+      Flowable__BaseUrl: http://flowable-rest:8080/flowable-rest/
+      Flowable__Username: rest-admin
+      Flowable__Password: test
+      Acl__BaseUrl: http://acl:8080/
+    ports:
+      - "8130:8080"
+    healthcheck:
+      test: ["CMD", "curl", "-fsS", "http://localhost:8080/health"]
+      interval: 5s
+      timeout: 3s
+      retries: 5
+      start_period: 10s
+    depends_on:
+      acl:
+        condition: service_healthy
+      flowable-init:
+        condition: service_completed_successfully
+    networks: [cg]
+
   # ── BFF ──────────────────────────────────────────────────────────────────
   bff:
     build:
diff --git a/infra/openzaak/seed_catalogus.py b/infra/openzaak/seed_catalogus.py
index a9aa4c6..5dc15a8 100644
--- a/infra/openzaak/seed_catalogus.py
+++ b/infra/openzaak/seed_catalogus.py
@@ -210,6 +210,10 @@ def main():
     print(f"zaaktypen in BIG: {names}")
     assert "BIG-REGISTRATIE" in names, "BIG-REGISTRATIE not listed"
     state = "published" if PUBLISH else "concept"
+    # Machine-readable line so callers (e.g. infra/run-domain-check.sh) can capture the
+    # zaaktype URL to configure the ACL's default-fill (ADR-0003/0009).
+    zt_url = next(z["url"] for z in zaaktypen if z.get("identificatie") == "BIG-REGISTRATIE")
+    print(f"ZAAKTYPE_URL {zt_url}")
     print(f"OK — BIG catalogus seeded (BIG-REGISTRATIE {state} + bsn eigenschap)")
 
 
diff --git a/infra/run-domain-check.sh b/infra/run-domain-check.sh
new file mode 100755
index 0000000..33d7d9e
--- /dev/null
+++ b/infra/run-domain-check.sh
@@ -0,0 +1,68 @@
+#!/usr/bin/env bash
+#
+# Verify the BIG Domain Service end-to-end (S-05) against an ALREADY-RUNNING full stack:
+# domain → Flowable (start the registratie process + external-task worker) → ACL → OpenZaak.
+# Submits a registration to the domain and asserts the worker opens a zaak in OpenZaak and
+# records its URL on the aggregate (ADR-0009).
+#
+# The seeded zaaktype URL is server-assigned, so it isn't knowable at initial bring-up. This
+# script therefore seeds a published BIG zaaktype and recreates the `acl` service configured to
+# default-fill it — pointing the ACL at the SAME OpenZaak host that owns the URL, so zaak creation
+# is host-consistent (exactly the configuration the ACL integration test proves, ADR-0006). That
+# one recreate aside, the caller owns stack bring-up + teardown.
+#
+# All in-network, reaching services by container IP (a single-label host isn't URL-valid; the
+# runner can't reach published ports — gitea-actions-gotchas.md §5/§6). Plain docker primitives.
+set -euo pipefail
+
+here="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+root="$(cd "$here/.." && pwd)"
+compose="$root/infra/docker-compose.yml"
+
+ip() { docker inspect -f '{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}' "$1"; }
+
+oz="$(docker ps -q --filter 'name=[-_]openzaak[-_]' | head -1)"
+dom="$(docker ps -q --filter 'name=domain' | head -1)"
+[ -n "$oz" ] || { echo "ERROR: no running OpenZaak container — bring the stack up first" >&2; exit 1; }
+[ -n "$dom" ] || { echo "ERROR: no running domain container — bring the stack up first" >&2; exit 1; }
+net="$(docker inspect -f '{{range $k,$_ := .NetworkSettings.Networks}}{{$k}}{{"\n"}}{{end}}' "$oz" | head -1)"
+oz_ip="$(ip "$oz")"; dom_ip="$(ip "$dom")"
+oz_base="http://$oz_ip:8000"
+echo ">> openzaak=$oz_ip domain=$dom_ip network=$net"
+
+echo ">> seeding a published BIG zaaktype (idempotent) and capturing its URL"
+sid="$(docker create --network "$net" -e "OZ_BASE=$oz_base" -e OZ_PUBLISH=1 python:3-slim python /seed.py)"
+docker cp "$here/openzaak/seed_catalogus.py" "$sid:/seed.py" >/dev/null
+zt_url="$(docker start -a "$sid" | sed -n 's/^ZAAKTYPE_URL //p' | head -1)"
+docker rm -f "$sid" >/dev/null
+[ -n "$zt_url" ] || { echo "ERROR: seed did not report a ZAAKTYPE_URL" >&2; exit 1; }
+echo ">> zaaktype: $zt_url"
+
+echo ">> recreating the acl service pointed at the seeded zaaktype (host-consistent)"
+ACL_ZAAKTYPE_URL="$zt_url" ACL_OPENZAAK_BASEURL="$oz_base/" docker compose -f "$compose" up -d acl
+WAIT_TIMEOUT="${WAIT_TIMEOUT:-120}" bash "$here/wait-healthy.sh" acl
+
+echo ">> submitting a registration to the domain"
+loc="$(docker run --rm --network "$net" curlimages/curl:latest \
+  -fsS -D - -o /dev/null -X POST "http://$dom_ip:8080/registrations" \
+  -H 'Content-Type: application/json' -d '{"bsn":"123456782"}' \
+  | sed -n 's/\r$//; s/^[Ll]ocation: //p' | head -1)"
+[ -n "$loc" ] || { echo "ERROR: POST /registrations returned no Location" >&2; exit 1; }
+echo ">> registration accepted at $loc"
+
+echo ">> polling the domain until the worker records the opened zaak"
+for _ in $(seq 1 30); do
+  body="$(docker run --rm --network "$net" curlimages/curl:latest \
+    -fsS "http://$dom_ip:8080$loc" 2>/dev/null || true)"
+  if echo "$body" | grep -q '/zaken/api/v1/zaken/'; then
+    echo "OK — the domain opened a zaak and recorded it on the registration:"
+    echo "$body" | cut -c1-300
+    exit 0
+  fi
+  sleep 2
+done
+echo "FAIL — the registration never received a zaak URL" >&2
+echo "--- domain log ---" >&2; docker logs "$dom" 2>&1 | tail -15 >&2
+acl="$(docker ps -q --filter 'name=[-_]acl[-_]' | head -1)"
+[ -n "$acl" ] && { echo "--- acl log ---" >&2; docker logs "$acl" 2>&1 | tail -15 >&2; }
+exit 1
diff --git a/services/domain/.dockerignore b/services/domain/.dockerignore
new file mode 100644
index 0000000..173dc20
--- /dev/null
+++ b/services/domain/.dockerignore
@@ -0,0 +1,4 @@
+**/bin
+**/obj
+**/*.user
+StrykerOutput
diff --git a/services/domain/Dockerfile b/services/domain/Dockerfile
new file mode 100644
index 0000000..600799b
--- /dev/null
+++ b/services/domain/Dockerfile
@@ -0,0 +1,34 @@
+# Multi-stage build for the BIG Domain Service (.NET 10).
+# Build context is services/domain (see infra/docker-compose.yml).
+FROM mcr.microsoft.com/dotnet/sdk:10.0 AS build
+WORKDIR /src
+
+# Restore first (cached unless .csproj files change).
+COPY Big.Api/Big.Api.csproj Big.Api/
+COPY Big.Application/Big.Application.csproj Big.Application/
+COPY Big.Infrastructure/Big.Infrastructure.csproj Big.Infrastructure/
+COPY Big.Domain/Big.Domain.csproj Big.Domain/
+RUN dotnet restore Big.Api/Big.Api.csproj
+
+COPY Big.Api/ Big.Api/
+COPY Big.Application/ Big.Application/
+COPY Big.Infrastructure/ Big.Infrastructure/
+COPY Big.Domain/ Big.Domain/
+RUN dotnet publish Big.Api/Big.Api.csproj -c Release -o /app/publish /p:UseAppHost=false
+
+FROM mcr.microsoft.com/dotnet/aspnet:10.0 AS runtime
+WORKDIR /app
+
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends curl \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY --from=build /app/publish .
+
+ENV ASPNETCORE_URLS=http://+:8080
+EXPOSE 8080
+
+HEALTHCHECK --interval=5s --timeout=3s --start-period=10s --retries=5 \
+    CMD curl -fsS http://localhost:8080/health || exit 1
+
+ENTRYPOINT ["dotnet", "Big.Api.dll"]