From 240d7b102513e8d9430d760b0add12d24184a20e Mon Sep 17 00:00:00 2001
From: Jun-te Kim <juntekim@googlemail.com>
Date: Fri, 19 Jun 2026 12:07:32 +0000
Subject: [PATCH] =?UTF-8?q?Resolve=20a=20landlord=20mains-gas=20override?=
 =?UTF-8?q?=20to=20the=20primary=20fuel=20code=20=F0=9F=9F=A5?=
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Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 .../property_overlays/main_fuel_overlay.py    | 22 +++++++++++++++++++
 tests/domain/epc/test_main_fuel_overlay.py    | 19 ++++++++++++++++
 2 files changed, 41 insertions(+)
 create mode 100644 domain/epc/property_overlays/main_fuel_overlay.py
 create mode 100644 tests/domain/epc/test_main_fuel_overlay.py

diff --git a/domain/epc/property_overlays/main_fuel_overlay.py b/domain/epc/property_overlays/main_fuel_overlay.py
new file mode 100644
index 00000000..4d721865
--- /dev/null
+++ b/domain/epc/property_overlays/main_fuel_overlay.py
@@ -0,0 +1,22 @@
+"""Map a Landlord-Override main-fuel value to a heating Simulation Overlay.
+
+A main-fuel value is one canonical gov-EPC `main_fuel` description ("mains gas",
+"electricity", …). The calculator reads the dwelling's primary fuel from
+`main_heating_details[0].main_fuel_type` as the RdSAP **int code**, so the
+overlay decomposes the value into that code and emits a whole-dwelling
+`HeatingOverlay` (fuel is not a per-building-part attribute, so `building_part`
+is ignored). Codes follow the modern RdSAP-20/21 `(not community)` family the
+gov-EPC API baseline uses. Unresolvable values produce no overlay.
+"""
+
+from __future__ import annotations
+
+from typing import Optional
+
+from domain.modelling.simulation import EpcSimulation
+
+
+def fuel_overlay_for(
+    main_fuel_value: str, building_part: int
+) -> Optional[EpcSimulation]:
+    raise NotImplementedError
diff --git a/tests/domain/epc/test_main_fuel_overlay.py b/tests/domain/epc/test_main_fuel_overlay.py
new file mode 100644
index 00000000..96072c1d
--- /dev/null
+++ b/tests/domain/epc/test_main_fuel_overlay.py
@@ -0,0 +1,19 @@
+"""The Landlord-Override main-fuel → heating Simulation Overlay mapping.
+
+A main-fuel value resolves to the RdSAP `main_fuel_type` int code the calculator
+reads from the dwelling's primary heating system; the overlay is whole-dwelling.
+"""
+
+from __future__ import annotations
+
+from domain.epc.property_overlays.main_fuel_overlay import fuel_overlay_for
+
+
+def test_mains_gas_overlays_the_primary_fuel() -> None:
+    # Act
+    simulation = fuel_overlay_for("mains gas", 0)
+
+    # Assert — mains gas (not community) is RdSAP main_fuel code 26.
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.main_fuel_type == 26