diff --git a/tests/domain/modelling/test_overlay_applicator.py b/tests/domain/modelling/test_overlay_applicator.py
index 64a237d7..2e7059ec 100644
--- a/tests/domain/modelling/test_overlay_applicator.py
+++ b/tests/domain/modelling/test_overlay_applicator.py
@@ -55,6 +55,33 @@ def test_apply_writes_targeted_building_part_and_leaves_others_untouched() -> No
     )
 
 
+def test_override_targeting_a_part_absent_from_the_epc_is_skipped() -> None:
+    # A Landlord Override can reference a building part the lodged (or predicted)
+    # EPC never carried — e.g. an `extension_1` override on a property whose EPC
+    # has only `main`. The orphaned part is skipped, not crashed on, so the rest
+    # of the overlay still folds and the property still models.
+    # Arrange — build_epc() has MAIN + EXTENSION_1 but no EXTENSION_2.
+    baseline: EpcPropertyData = build_epc()
+    simulation = EpcSimulation(
+        building_parts={
+            BuildingPartIdentifier.MAIN: BuildingPartOverlay(wall_insulation_type=1),
+            BuildingPartIdentifier.EXTENSION_2: BuildingPartOverlay(
+                wall_insulation_type=1
+            ),
+        }
+    )
+
+    # Act
+    result: EpcPropertyData = apply_simulations(baseline, [simulation])
+
+    # Assert — the present part got its overlay; the absent one was not added.
+    assert _part(result, BuildingPartIdentifier.MAIN).wall_insulation_type == 1
+    assert all(
+        part.identifier is not BuildingPartIdentifier.EXTENSION_2
+        for part in result.sap_building_parts
+    )
+
+
 def test_flat_roof_construction_type_folds_onto_the_part() -> None:
     # ADR-0033: a flat-roof landlord override sets `roof_construction_type` so the
     # calculator's flat path (`"flat" in roof_construction_type`) fires the