diff --git a/docs/extending.md b/docs/extending.md
new file mode 100644
index 0000000..c363638
--- /dev/null
+++ b/docs/extending.md
@@ -0,0 +1,53 @@
+# Extending the framework (extension points)
+
+portfolio-optimiser is a generic core with explicit **config seams** (D4/D5, 90 %-prinsippet):
+you onboard a new project, a new data source, or a new model-map **without editing the core
+`src/portfolio_optimiser/*.py`**. The three guides below name the exact seam for each.
+
+> **Honesty note (rent teknisk rammeverk).** The bundled reference domain
+> (`data/reference_projects.json` + `data/docs/<id>/`) is a set of **SYNTHETIC, AI-authored
+> fixtures** — fictional construction-cost projects, dummy estimates, and placeholder
+> `verdict_input` decisions. They are flagged in each file's `_note`. A production deployer
+> **replaces the data source** with their own and supplies **Layer-2 verdicts via real HITL**
+> (fageksperter), not static config. The static `verdict_input` field is a test-fixture
+> convenience that stands in for the durable HITL verdict in the offline synthetic framework.
+
+## Legg til eget prosjekt
+
+A new project is **config + docs only** — no code change (this is exercised by the SC1 test
+`test_e_new_project_flows_through_via_config_only` + the `test_f_no_hardcoded_project_ids_in_src`
+guard, which fails if any project id leaks into `src/`).
+
+1. Append an entry to `src/portfolio_optimiser/data/reference_projects.json` with the full key
+   set: `id`, `name`, `description`, `currency`, `cost_items`, **`docs_dir`**, and
+   **`verdict_input`** (`{"decision", "rationale"}`).
+   - `docs_dir` is a path **relative to the package `data/` root** (e.g. `"docs/MY-PROJECT"`);
+     the loader (`reference_domain.load_reference_projects`) resolves it to an absolute path.
+   - `verdict_input` carries the (synthetic) Layer-2 decision/rationale; flag it in the file's
+     `_note` as synthetic if it is not a real expert verdict.
+2. Create the bundled docs folder `src/portfolio_optimiser/data/docs/<id>/` with at least one
+   text file whose content names the cost-saving measure/terms (so `retrieve_chunks` returns at
+   least one citable chunk).
+3. Run the project: `run_portfolio(["MY-PROJECT"], "local", client_factory=...)` (or include it
+   in the default fan-out by passing no `project_ids`).
+
+## Legg til egen datakilde
+
+The retriever (`retrieval.py` / `datasource.py`) reads a **local docs folder** per project,
+selected by the project's `docs_dir` in `reference_projects.json`. To point a project at your own
+data, change its `docs_dir` to your folder and drop your cost documentation there — the citation
+seam (`{file, locator, snippet, score}`) is identical on the in-process tool and MCP paths. The
+folder is boundary-checked (fail-closed) against path traversal, so keep documents inside the
+configured `docs_dir`. A real deployer swaps the bundled synthetic docs for their own source.
+
+## Legg til egen modell-map
+
+Model choice is **config, not code** (B12): `src/portfolio_optimiser/data/model_map.json` maps
+`profile -> role -> model/deployment` (`resolve_model(profile, role)`). To use your own models:
+
+- Edit the `local` block to your local model ids (Ollama/LM Studio), and/or
+- Edit the `azure` block to your Foundry deployment names (the placeholders
+  `REPLACE-WITH-FOUNDRY-DEPLOYMENT` are tenant-specific — replace them or supply via env).
+
+The role keys (`proposer`, `checker`, `default`) let you assign a distinct model per debate role;
+`default` is the fallback when a role is unmapped.
diff --git a/tests/test_portfolio.py b/tests/test_portfolio.py
index 7380162..544cc45 100644
--- a/tests/test_portfolio.py
+++ b/tests/test_portfolio.py
@@ -211,3 +211,16 @@ def test_f_no_hardcoded_project_ids_in_src() -> None:
         if pid in py.read_text(encoding="utf-8")
     ]
     assert offenders == [], f"hardcoded project ids in src: {offenders}"
+
+
+def test_g_extension_doc_exists_and_references_seams() -> None:
+    """SC6: the extension guide exists and names the concrete config seams a deployer extends —
+    the per-project config (`reference_projects.json` with `docs_dir` + `verdict_input`) and the
+    `model_map.json` role->deployment map. Pattern: test_budget.py:88-99 (file-content guard)."""
+    from pathlib import Path
+
+    doc = Path("docs/extending.md")
+    assert doc.exists(), "docs/extending.md is missing"
+    text = doc.read_text(encoding="utf-8")
+    for seam in ("reference_projects.json", "docs_dir", "verdict_input", "model_map.json"):
+        assert seam in text, f"extension doc does not reference seam: {seam}"