diff --git a/packages/README.md b/packages/README.md
new file mode 100644
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--- /dev/null
+++ b/packages/README.md
@@ -0,0 +1,16 @@
+# Shared packages
+
+Workspace packages consumed by `services/*`. Each package is its own Python distribution with its own `pyproject.toml`; services import via the workspace dependency mechanism (`{ workspace = true }`).
+
+| Package | Purpose |
+|---------|---------|
+| [`domain/`](./domain/) | Shared domain types — `Property`, `BaselinePerformance`, `Plan`, `Scenario`, `EpcPropertyData`, etc. No persistence, no IO, no business logic. |
+| [`repos/`](./repos/) | Persistence layer — one repo per aggregate. Owns the SQL. Depends on `domain`. |
+| [`fetchers/`](./fetchers/) | External API clients (gov EPC, Ofgem, Google Solar, etc.). Depend on `domain` for response shapes. |
+| [`utils/`](./utils/) | Cross-cutting infra — logging, S3, CloudWatch URL builders, SQS task helpers. |
+
+## Adding a new shared package
+
+Only when a real second consumer materialises. Don't pre-shatter (`repos-epc`, `repos-property`, ...) — split when a deployment needs to drop a dep, not before.
+
+See [`../ara_backend_design.md`](../ara_backend_design.md) §11 for the broader monorepo layout and [`../CONTEXT.md`](../CONTEXT.md) for the domain glossary that names the types living in `domain/`.
diff --git a/packages/domain/README.md b/packages/domain/README.md
new file mode 100644
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--- /dev/null
+++ b/packages/domain/README.md
@@ -0,0 +1,30 @@
+# domna-domain
+
+Shared domain types — `Property`, `Properties`, `BaselinePerformance`, `Plan`, `PlanPhase`, `Scenario`, `ScenarioPhase`, `ScenarioSnapshot`, `Recommendation`, `OptimisedPackage`, `EpcPropertyData`, etc.
+
+**Boundary**: types only. No persistence, no IO, no business logic. Other packages and services depend on `domna-domain`; this package depends on nothing internal.
+
+Domain definitions live in [`../../CONTEXT.md`](../../CONTEXT.md). New types added here must match the glossary terms.
+
+## Layout
+
+```
+src/domain/
+├── __init__.py
+├── property.py             # Property, Properties, PropertyIdentity
+├── site_notes.py
+├── landlord_overrides.py
+├── baseline_performance.py # lodged + effective pair (ADR-0004)
+├── plan.py                 # Plan, PlanPhase, OptimisedPackage
+├── scenario.py             # Scenario, ScenarioPhase, ScenarioSnapshot (ADR-0005)
+├── recommendation.py
+├── geospatial.py
+├── solar.py
+├── anomaly_flags.py
+└── ml/
+    ├── __init__.py
+    ├── transform.py        # EpcMlTransform (versioned per §8.3)
+    └── schema.py
+```
+
+When `datatypes/epc/domain/` folds in, the EPC schema types move under `src/domain/epc/`.
diff --git a/packages/domain/pyproject.toml b/packages/domain/pyproject.toml
new file mode 100644
index 00000000..5e820371
--- /dev/null
+++ b/packages/domain/pyproject.toml
@@ -0,0 +1,13 @@
+[project]
+name = "domna-domain"
+version = "0.1.0"
+description = "Shared domain types for the Ara modelling pipeline and sibling Domna services."
+requires-python = ">=3.11"
+dependencies = []
+
+[build-system]
+requires = ["hatchling"]
+build-backend = "hatchling.build"
+
+[tool.hatch.build.targets.wheel]
+packages = ["src/domain"]
diff --git a/packages/domain/src/domain/__init__.py b/packages/domain/src/domain/__init__.py
new file mode 100644
index 00000000..1d52198c
--- /dev/null
+++ b/packages/domain/src/domain/__init__.py
@@ -0,0 +1,4 @@
+"""Shared domain types for the Ara modelling pipeline and sibling Domna services.
+
+No persistence, no IO, no business logic. See README.md for layout.
+"""
diff --git a/packages/fetchers/README.md b/packages/fetchers/README.md
new file mode 100644
index 00000000..ebe47f74
--- /dev/null
+++ b/packages/fetchers/README.md
@@ -0,0 +1,19 @@
+# domna-fetchers
+
+External API clients. Each fetcher is responsible for one external system — `EpcClientService` for the gov EPC API, `GeospatialFetcher` for Ordnance Survey, `SolarFetcher` for Google Solar, `FuelRatesFetcher` for Ofgem, `CarbonFactorsFetcher` for Defra.
+
+**Boundary**: makes HTTP calls + returns raw or lightly-mapped responses. No DB, no business logic. Modelling services never depend on fetchers — only orchestrators do (per [ADR-0003](../../docs/adr/0003-strict-ingestion-modelling-separation.md)).
+
+## Layout
+
+```
+src/fetchers/
+├── __init__.py
+├── epc_client.py          # wraps backend/epc_client/
+├── geospatial.py
+├── solar.py
+├── fuel_rates_fetcher.py
+└── carbon_factors_fetcher.py
+```
+
+`backend/epc_client/` will fold into `epc_client.py` during the migration; until then this module re-exports from the legacy location.
diff --git a/packages/fetchers/pyproject.toml b/packages/fetchers/pyproject.toml
new file mode 100644
index 00000000..69404681
--- /dev/null
+++ b/packages/fetchers/pyproject.toml
@@ -0,0 +1,19 @@
+[project]
+name = "domna-fetchers"
+version = "0.1.0"
+description = "External API clients — gov EPC, Ofgem, Google Solar, Defra, etc."
+requires-python = ">=3.11"
+dependencies = [
+    "domna-domain",
+    "httpx>=0.27",
+]
+
+[tool.uv.sources]
+domna-domain = { workspace = true }
+
+[build-system]
+requires = ["hatchling"]
+build-backend = "hatchling.build"
+
+[tool.hatch.build.targets.wheel]
+packages = ["src/fetchers"]
diff --git a/packages/fetchers/src/fetchers/__init__.py b/packages/fetchers/src/fetchers/__init__.py
new file mode 100644
index 00000000..74646907
--- /dev/null
+++ b/packages/fetchers/src/fetchers/__init__.py
@@ -0,0 +1,4 @@
+"""External API clients for Ara and sibling services.
+
+One fetcher per external system. No DB, no business logic. See README.md.
+"""
diff --git a/packages/repos/README.md b/packages/repos/README.md
new file mode 100644
index 00000000..990b66db
--- /dev/null
+++ b/packages/repos/README.md
@@ -0,0 +1,27 @@
+# domna-repos
+
+Persistence layer. One repo per aggregate; owns the SQL for its tables. Callers see only domain objects from `domna-domain`.
+
+**Boundary**: depends on `domna-domain` for types. No external IO except the DB. No business logic — services do that.
+
+## Repos (per [PRD §7.3](../../ara_backend_design.md))
+
+```
+src/repos/
+├── __init__.py
+├── unit_of_work.py
+├── property_repo.py
+├── epc_cache_repo.py
+├── site_notes_repo.py
+├── landlord_overrides_repo.py
+├── recommendations_repo.py
+├── generic_data_repo.py
+├── fuel_rates_repo.py
+├── carbon_factors_repo.py
+├── heating_system_assumptions_repo.py
+└── subtask_repo.py
+```
+
+Each repo has a `Fake*Repo` companion in its service's test tree (typically `services/ara/tests/fakes/`) — dict-backed, no DB.
+
+DDL migrations are scoped to sub-PRD (iii); during Phase 0 repos may delegate into the legacy `backend/app/db/db_funcs.*` modules.
diff --git a/packages/repos/pyproject.toml b/packages/repos/pyproject.toml
new file mode 100644
index 00000000..53689812
--- /dev/null
+++ b/packages/repos/pyproject.toml
@@ -0,0 +1,19 @@
+[project]
+name = "domna-repos"
+version = "0.1.0"
+description = "Persistence layer — one repo per aggregate. Owns the SQL."
+requires-python = ">=3.11"
+dependencies = [
+    "domna-domain",
+    "sqlalchemy>=2.0",
+]
+
+[tool.uv.sources]
+domna-domain = { workspace = true }
+
+[build-system]
+requires = ["hatchling"]
+build-backend = "hatchling.build"
+
+[tool.hatch.build.targets.wheel]
+packages = ["src/repos"]
diff --git a/packages/repos/src/repos/__init__.py b/packages/repos/src/repos/__init__.py
new file mode 100644
index 00000000..a981395a
--- /dev/null
+++ b/packages/repos/src/repos/__init__.py
@@ -0,0 +1,4 @@
+"""Persistence layer for the Ara domain aggregates.
+
+One repo per aggregate. Owns SQL; exposes domain objects. See README.md.
+"""
diff --git a/packages/utils/README.md b/packages/utils/README.md
new file mode 100644
index 00000000..1fba6457
--- /dev/null
+++ b/packages/utils/README.md
@@ -0,0 +1,15 @@
+# domna-utils
+
+Cross-cutting infrastructure helpers. Nothing domain-specific — anything in here should be portable across services.
+
+## Will live here (migrating from `utils/` and `backend/utils/`)
+
+- Logging — `logger.py`
+- S3 — `s3.py`
+- Pandas helpers — `pandas_utils.py`
+- CloudWatch URL builder — `cloudwatch.py`
+- SQS subtask helpers — `subtasks.py`
+
+## Will NOT live here
+
+Service-specific parsers (Osmosis condition report, full-SAP parser, SharePoint integration) move into the service that owns them, not here.
diff --git a/packages/utils/pyproject.toml b/packages/utils/pyproject.toml
new file mode 100644
index 00000000..cf739bbd
--- /dev/null
+++ b/packages/utils/pyproject.toml
@@ -0,0 +1,15 @@
+[project]
+name = "domna-utils"
+version = "0.1.0"
+description = "Cross-cutting infrastructure helpers — logging, S3, CloudWatch, SQS tasks."
+requires-python = ">=3.11"
+dependencies = [
+    "boto3>=1.34",
+]
+
+[build-system]
+requires = ["hatchling"]
+build-backend = "hatchling.build"
+
+[tool.hatch.build.targets.wheel]
+packages = ["src/utils"]
diff --git a/packages/utils/src/utils/__init__.py b/packages/utils/src/utils/__init__.py
new file mode 100644
index 00000000..d010a3be
--- /dev/null
+++ b/packages/utils/src/utils/__init__.py
@@ -0,0 +1,4 @@
+"""Cross-cutting infrastructure helpers — logging, S3, CloudWatch, SQS tasks.
+
+Nothing domain-specific belongs here. See README.md.
+"""
diff --git a/pyproject.toml b/pyproject.toml
index 49108861..75aabc82 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1 +1,13 @@
 [tool.pyright]
+
+# uv workspace root.
+# Each workspace member has its own pyproject.toml under packages/<name>/ or services/<name>/.
+# Run `uv sync` at the root to install everything; `uv sync --package <name>` for one.
+[tool.uv.workspace]
+members = [
+    "packages/domain",
+    "packages/repos",
+    "packages/fetchers",
+    "packages/utils",
+    "services/ara",
+]
diff --git a/services/README.md b/services/README.md
new file mode 100644
index 00000000..c82ef6a4
--- /dev/null
+++ b/services/README.md
@@ -0,0 +1,13 @@
+# Services
+
+Each subdirectory is a deployable unit — typically a Lambda image. Own `pyproject.toml`, own `Dockerfile`, own deps. Lambda bundle contains only that service's deps + its workspace deps.
+
+| Service | Purpose |
+|---------|---------|
+| [`ara/`](./ara/) | The Domna retrofit modelling backend — ingestion + modelling pipelines, all 9 services in [PRD §9.2](../ara_backend_design.md). |
+
+Other Domna services (address2uprn, hubspot, pashub, ecmk, magicplan) live in the legacy `backend/` and `etl/` trees for now; they are slated to migrate here as their owners pick them up — see [PRD §11](../ara_backend_design.md). When that work starts, scaffold the service under `services/<name>/` and add it to the workspace members in the root `pyproject.toml`.
+
+## Service boundary
+
+A service can `import domain.*`, `import repos.*`, `import fetchers.*`, `import utils.*` (workspace deps). It **cannot** import another service's modules — they are separate distributions with no cross-import path. This is the structural enforcement of the modelling/ingestion separation ([ADR-0003](../docs/adr/0003-strict-ingestion-modelling-separation.md)).
diff --git a/services/ara/Dockerfile b/services/ara/Dockerfile
new file mode 100644
index 00000000..c45d6bc1
--- /dev/null
+++ b/services/ara/Dockerfile
@@ -0,0 +1,12 @@
+# Lambda image for the Ara modelling backend.
+#
+# This is a scaffold — final image will install only ara + its workspace deps
+# (domna-domain, domna-repos, domna-fetchers, domna-utils) plus ML/data libraries.
+# Build via uv to keep cold-start size contained.
+
+FROM public.ecr.aws/lambda/python:3.11
+
+# TODO: install uv, sync this service's deps from the workspace lock file,
+# copy src/ara/ into ${LAMBDA_TASK_ROOT}/, set CMD to the Lambda handler.
+
+CMD ["ara.lambdas.handler.handler"]
diff --git a/services/ara/README.md b/services/ara/README.md
new file mode 100644
index 00000000..71e71a5d
--- /dev/null
+++ b/services/ara/README.md
@@ -0,0 +1,30 @@
+# ara
+
+The Domna retrofit modelling backend. Replaces the legacy `backend/engine/engine.py` monolith with a service-oriented pipeline that survives the 30 May 2026 gov EPC API cut-over and that other team members can read, fix, and extend.
+
+Design document: [`../../ara_backend_design.md`](../../ara_backend_design.md).
+Domain glossary: [`../../CONTEXT.md`](../../CONTEXT.md).
+
+## Layout
+
+```
+src/ara/
+├── services/        # the 9 domain services from PRD §9.2:
+│                    #   EpcRemappingService, EpcPredictionService,
+│                    #   FeatureBuilder, EpcEnergyDerivationService,
+│                    #   RebaseliningService, RecommendationService,
+│                    #   ImpactPredictionService, OptimiserService,
+│                    #   ValuationService, ResultsPersister
+├── orchestrators/   # IngestionPipeline, ModellingPipeline, RefreshOrchestrator
+└── lambdas/         # one handler.py per Lambda + the event-shape contracts
+```
+
+## Pipeline
+
+See [PRD §9.4](../../ara_backend_design.md) for the per-batch step order. Briefly: per-property setup (steps 1–6) runs once per Property; the per-scenario × per-phase loop (steps 7–10) re-derives candidates and impact predictions against the rolling Effective EPC state; results are persisted under one Unit of Work per (Plan, Scenario).
+
+## Testing
+
+- `tests/unit/` — service tests against fakes from `tests/fakes/`. No DB, no network, no ML lambda.
+- `tests/integration/` — real Postgres (testcontainers / localstack), fake fetchers + fake ML lambdas.
+- ML transform contract tests live with `domain.ml.transform` in `packages/domain/`.
diff --git a/services/ara/pyproject.toml b/services/ara/pyproject.toml
new file mode 100644
index 00000000..3556a15f
--- /dev/null
+++ b/services/ara/pyproject.toml
@@ -0,0 +1,28 @@
+[project]
+name = "ara"
+version = "0.1.0"
+description = "The Domna retrofit modelling backend. Ingestion + modelling pipelines."
+requires-python = ">=3.11"
+dependencies = [
+    "domna-domain",
+    "domna-repos",
+    "domna-fetchers",
+    "domna-utils",
+    "pandas>=2.0",
+    "pandas-stubs",
+    "numpy>=1.26",
+    "pydantic>=2.0",
+]
+
+[tool.uv.sources]
+domna-domain = { workspace = true }
+domna-repos = { workspace = true }
+domna-fetchers = { workspace = true }
+domna-utils = { workspace = true }
+
+[build-system]
+requires = ["hatchling"]
+build-backend = "hatchling.build"
+
+[tool.hatch.build.targets.wheel]
+packages = ["src/ara"]
diff --git a/services/ara/src/ara/__init__.py b/services/ara/src/ara/__init__.py
new file mode 100644
index 00000000..26856c73
--- /dev/null
+++ b/services/ara/src/ara/__init__.py
@@ -0,0 +1,4 @@
+"""The Domna retrofit modelling backend.
+
+See README.md and ara_backend_design.md (repo root) for the architecture.
+"""
diff --git a/services/ara/src/ara/lambdas/__init__.py b/services/ara/src/ara/lambdas/__init__.py
new file mode 100644
index 00000000..93b08582
--- /dev/null
+++ b/services/ara/src/ara/lambdas/__init__.py
@@ -0,0 +1,5 @@
+"""Lambda handlers + event-shape contracts.
+
+One handler per deployable Lambda. See PRD §4.6 for the ModelTriggerRequest
+shape.
+"""
diff --git a/services/ara/src/ara/orchestrators/__init__.py b/services/ara/src/ara/orchestrators/__init__.py
new file mode 100644
index 00000000..4d2c9a60
--- /dev/null
+++ b/services/ara/src/ara/orchestrators/__init__.py
@@ -0,0 +1,5 @@
+"""Orchestrators for the Ara pipeline.
+
+IngestionPipeline, ModellingPipeline, RefreshOrchestrator. The only place
+where step order is encoded and where fetchers + services + repos meet.
+"""
diff --git a/services/ara/src/ara/services/__init__.py b/services/ara/src/ara/services/__init__.py
new file mode 100644
index 00000000..b561f336
--- /dev/null
+++ b/services/ara/src/ara/services/__init__.py
@@ -0,0 +1,9 @@
+"""Domain services for the Ara modelling pipeline (PRD §9.2).
+
+EpcRemappingService, EpcPredictionService, FeatureBuilder,
+EpcEnergyDerivationService, RebaseliningService, RecommendationService,
+ImpactPredictionService, OptimiserService, ValuationService, ResultsPersister.
+
+Each service operates on `Properties` and depends only on repos + other services
++ domain objects. No external IO (per ADR-0003).
+"""
diff --git a/services/ara/tests/__init__.py b/services/ara/tests/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/services/ara/tests/fakes/__init__.py b/services/ara/tests/fakes/__init__.py
new file mode 100644
index 00000000..cc032044
--- /dev/null
+++ b/services/ara/tests/fakes/__init__.py
@@ -0,0 +1,4 @@
+"""Fake repos and fetchers for unit tests.
+
+One Fake<Name>Repo per real repo; dict-backed; no DB. Same for fetchers.
+"""
diff --git a/services/ara/tests/integration/__init__.py b/services/ara/tests/integration/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/services/ara/tests/unit/__init__.py b/services/ara/tests/unit/__init__.py
new file mode 100644
index 00000000..e69de29b