diff --git a/tests/repositories/property/test_landlord_override_overlays.py b/tests/repositories/property/test_landlord_override_overlays.py
index c3f30fc3d..e6e60b4e3 100644
--- a/tests/repositories/property/test_landlord_override_overlays.py
+++ b/tests/repositories/property/test_landlord_override_overlays.py
@@ -72,6 +72,46 @@ def test_resolve_boiler_archetype_leaves_other_cases_unchanged(
     assert resolved == heating_value
 
 
+def test_gas_boiler_override_on_oil_fuel_builds_the_oil_boiler_overlay() -> None:
+    # ADR-0067: a fuel-agnostic gas-boiler override sitting on an oil `main_fuel`
+    # must resolve to the oil boiler at overlay time (the join reads the sibling
+    # fuel row), so it scores the oil boiler code (127), not the gas default (102).
+
+    # Arrange
+    overrides = ResolvedPropertyOverrides(
+        rows=(
+            ResolvedPropertyOverride("main_heating_system", 0, "Gas boiler, regular"),
+            ResolvedPropertyOverride("main_fuel", 0, "oil"),
+        )
+    )
+
+    # Act
+    overlays = overlays_from(overrides)
+
+    # Assert — the heating overlay carries the oil boiler code.
+    heating = next(o.heating for o in overlays if o.heating is not None)
+    assert heating.sap_main_heating_code == 127
+
+
+def test_gas_boiler_override_on_gas_fuel_stays_a_gas_boiler() -> None:
+    # The join only fires for non-gas fuels; a genuine gas boiler is untouched.
+
+    # Arrange
+    overrides = ResolvedPropertyOverrides(
+        rows=(
+            ResolvedPropertyOverride("main_heating_system", 0, "Gas boiler, regular"),
+            ResolvedPropertyOverride("main_fuel", 0, "mains gas"),
+        )
+    )
+
+    # Act
+    overlays = overlays_from(overrides)
+
+    # Assert — the gas regular condensing code (102).
+    heating = next(o.heating for o in overlays if o.heating is not None)
+    assert heating.sap_main_heating_code == 102
+
+
 def test_roof_type_row_produces_a_roof_overlay() -> None:
     # Arrange
     overrides = ResolvedPropertyOverrides(