diff --git a/domain/modelling/scoring/overlay_applicator.py b/domain/modelling/scoring/overlay_applicator.py
index 961f4e02..6a50fde4 100644
--- a/domain/modelling/scoring/overlay_applicator.py
+++ b/domain/modelling/scoring/overlay_applicator.py
@@ -41,7 +41,13 @@ def apply_simulations(
 
     for simulation in simulations:
         for identifier, overlay in simulation.building_parts.items():
-            part = parts_by_id[identifier]
+            # A Landlord Override can target a building part the lodged (or
+            # predicted) EPC never carried — e.g. an `extension_1` override on a
+            # property whose EPC has only `main`. Skip the orphaned part rather
+            # than crashing the whole property's modelling on a `KeyError`.
+            part = parts_by_id.get(identifier)
+            if part is None:
+                continue
             for overlay_field in fields(overlay):
                 value = getattr(overlay, overlay_field.name)
                 if value is not None: