diff --git a/src/newgrf.cpp b/src/newgrf.cpp
index 0b6130a72f..fd2e52e58f 100644
--- a/src/newgrf.cpp
+++ b/src/newgrf.cpp
@@ -1560,7 +1560,7 @@ static ChangeInfoResult TownHouseChangeInfo(uint hid, int numinfo, int prop, byt
 				break;
 
 			case 0x14: // House callback mask
-				housespec->callback_mask = grf_load_byte(&buf);
+				housespec->callback_mask |= grf_load_byte(&buf);
 				break;
 
 			case 0x15: { // House override byte