diff --git a/ship_cmd.c b/ship_cmd.c
--- a/ship_cmd.c
+++ b/ship_cmd.c
@@ -878,16 +878,17 @@ int32 CmdBuildShip(int x, int y, uint32 
 
 	if (!IsEngineBuildable(p1, VEH_Ship)) return CMD_ERROR;
 
-	if (!IsTileDepotType(tile, TRANSPORT_WATER)) return CMD_ERROR;
-
-	if (_map_owner[tile] != _current_player) return CMD_ERROR;
-
 	SET_EXPENSES_TYPE(EXPENSES_NEW_VEHICLES);
 
 	value = EstimateShipCost(p1);
 	if (flags & DC_QUERY_COST)
 		return value;
 
+	/* The ai_new queries the vehicle cost before building the route,
+	 * so we must check against cheaters no sooner than now. --pasky */
+	if (!IsTileDepotType(tile, TRANSPORT_WATER)) return CMD_ERROR;
+	if (_map_owner[tile] != _current_player) return CMD_ERROR;
+
 	v = AllocateVehicle();
 	if (v == NULL || IsOrderPoolFull() ||
 			(unit_num = GetFreeUnitNumber(VEH_Ship)) > _patches.max_ships)
diff --git a/train_cmd.c b/train_cmd.c
--- a/train_cmd.c
+++ b/train_cmd.c
@@ -542,9 +542,10 @@ int32 CmdBuildRailVehicle(int x, int y, 
 
 	if (!IsEngineBuildable(p1, VEH_Train)) return CMD_ERROR;
 
-	if (!IsTileDepotType(tile, TRANSPORT_RAIL)) return CMD_ERROR;
-
-	if (_map_owner[tile] != _current_player) return CMD_ERROR;
+	if (!(flags & DC_QUERY_COST)) {
+		if (!IsTileDepotType(tile, TRANSPORT_RAIL)) return CMD_ERROR;
+		if (_map_owner[tile] != _current_player) return CMD_ERROR;
+	}
 
 	_cmd_build_rail_veh_var1 = 0;