Index: town_cmd.c
===================================================================
--- town_cmd.c	(revision 3521)
+++ town_cmd.c	(working copy)
@@ -813,8 +813,8 @@
 	for (ptr = _town_coord_mod; ptr != endof(_town_coord_mod); ++ptr) {
 		FindLandscapeHeightByTile(&ti, tile);
 
-		// Only work with plain land that not already has a house with map5=0
-		if (ti.tileh == 0 && (ti.type != MP_HOUSE || ti.map5 != 0)) {
+		// Only work with flat land that not already has a finished house
+		if (ti.tileh == 0 && (ti.type != MP_HOUSE || GB(_m[ti.tile].m3, 6, 2) != 0x3)) {
 			if (!CmdFailed(DoCommandByTile(tile, 0, 0, DC_AUTO, CMD_LANDSCAPE_CLEAR))) {
 				DoCommandByTile(tile, GenRandomRoadBits(), t->index, DC_EXEC | DC_AUTO, CMD_BUILD_ROAD);
 				_current_player = old_player;