diff --git a/src/town.h b/src/town.h
--- a/src/town.h
+++ b/src/town.h
@@ -299,7 +299,7 @@ static inline HouseSpec *GetHouseSpecs(H
 	return &_house_specs[house_id];
 }
 
-TileIndex GetHouseNorthPart(HouseID &house);
+TileIndexDiff GetHouseNorthPart(HouseID &house);
 
 /**
  * Check if a TownID is valid.