From 79fe54a82289d5b4c2a374a41decf59d79a1b83b Mon Sep 17 00:00:00 2001
From: Khalim Conn-Kowlessar <kconnkowlessar@gmail.com>
Date: Tue, 30 Jun 2026 17:16:14 +0000
Subject: [PATCH] =?UTF-8?q?Log=20a=20landlord=20fuel=20that=20contradicts?=
 =?UTF-8?q?=20the=20heating=20archetype's=20natural=20fuel=20=F0=9F=9F=A9?=
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Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 .../main_heating_system_overlay.py            | 10 +++++
 .../property/landlord_override_overlays.py    | 38 +++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/domain/epc/property_overlays/main_heating_system_overlay.py b/domain/epc/property_overlays/main_heating_system_overlay.py
index b08b230cf..636fa6191 100644
--- a/domain/epc/property_overlays/main_heating_system_overlay.py
+++ b/domain/epc/property_overlays/main_heating_system_overlay.py
@@ -198,6 +198,16 @@ def _gas_boiler_overlay(code: int) -> HeatingOverlay:
     )
 
 
+def natural_fuel_for(main_heating_value: str) -> Optional[int]:
+    """The RdSAP `main_fuel` code a heating archetype unambiguously implies — its
+    natural fuel (ADR-0041) — or None for an unmapped archetype. Exposed so a
+    plausibility check can compare it against a landlord `main_fuel` override."""
+    code = _MAIN_HEATING_CODES.get(main_heating_value)
+    if code is None:
+        return None
+    return _natural_fuel_for(code)
+
+
 def main_heating_overlay_for(
     main_heating_value: str, building_part: int
 ) -> Optional[EpcSimulation]:
diff --git a/repositories/property/landlord_override_overlays.py b/repositories/property/landlord_override_overlays.py
index 7477cc785..5f47a3b39 100644
--- a/repositories/property/landlord_override_overlays.py
+++ b/repositories/property/landlord_override_overlays.py
@@ -28,6 +28,7 @@ values are live vs no-op before any write.
 
 from __future__ import annotations
 
+import logging
 from typing import Callable, Optional
 
 from domain.epc.property_overlays.attribute_overlay import (
@@ -41,6 +42,7 @@ from domain.epc.property_overlays.glazing_overlay import glazing_overlay_for
 from domain.epc.property_overlays.main_fuel_overlay import fuel_overlay_for
 from domain.epc.property_overlays.main_heating_system_overlay import (
     main_heating_overlay_for,
+    natural_fuel_for,
 )
 from domain.epc.property_overlays.water_heating_overlay import (
     water_heating_overlay_for,
@@ -50,6 +52,8 @@ from domain.epc.property_overlays.wall_type_overlay import wall_overlay_for
 from domain.modelling.simulation import EpcSimulation
 from repositories.property.property_overrides_reader import ResolvedPropertyOverrides
 
+logger = logging.getLogger(__name__)
+
 # Each override component maps its value (+ building part) to an overlay, or None
 # when the value isn't resolvable. Fabric (wall/roof) folds onto building parts;
 # property_type / built_form_type are whole-dwelling categorical corrections
@@ -90,3 +94,37 @@ def overlays_from(overrides: ResolvedPropertyOverrides) -> list[EpcSimulation]:
         if overlay is not None:
             overlays.append(overlay)
     return overlays
+
+
+def _override_value(overrides: ResolvedPropertyOverrides, component: str) -> Optional[str]:
+    for row in overrides.rows:
+        if row.override_component == component:
+            return row.override_value
+    return None
+
+
+def flag_fuel_mismatch(overrides: ResolvedPropertyOverrides) -> None:
+    """Log (not raise) when a landlord `main_fuel` override contradicts the
+    `main_heating_system` archetype's natural fuel — a plausibility signal (e.g.
+    a gas fuel on an electric room heater). The override is still honoured (it
+    wins, ADR-0041); we only record the implausibility. Deferred handling."""
+    heating_value = _override_value(overrides, "main_heating_system")
+    fuel_value = _override_value(overrides, "main_fuel")
+    if heating_value is None or fuel_value is None:
+        return
+    natural = natural_fuel_for(heating_value)
+    fuel_overlay = fuel_overlay_for(fuel_value, 0)
+    override_fuel = (
+        fuel_overlay.heating.main_fuel_type
+        if fuel_overlay is not None and fuel_overlay.heating is not None
+        else None
+    )
+    if natural is None or override_fuel is None:
+        return
+    if natural != override_fuel:
+        logger.warning(
+            "Landlord main_fuel %r contradicts the natural fuel of heating "
+            "system %r (override is honoured; flagged for review)",
+            fuel_value,
+            heating_value,
+        )