Index: src/station_cmd.cpp
===================================================================
--- src/station_cmd.cpp	2007-12-16 00:11:18.000000000 +0100
+++ src/station_cmd.cpp	2007-12-17 21:06:36.000000000 +0100
@@ -703,9 +703,10 @@
  * @param invalid_dirs prohibited directions
  * @param station StationID to be queried and returned if available
  * @param check_clear if clearing tile should be performed (in wich case, cost will be added)
+ * @param st station type to allow in search area
  * @return the cost in case of success, or an error code if it failed.
  */
-CommandCost CheckFlatLandBelow(TileIndex tile, uint w, uint h, uint flags, uint invalid_dirs, StationID *station, bool check_clear = true)
+CommandCost CheckFlatLandBelow(TileIndex tile, uint w, uint h, uint flags, uint invalid_dirs, StationID *station, bool check_clear = true, StationType st = STATION_RAIL)
 {
 	CommandCost cost;
 	int allowed_z = -1;
@@ -754,11 +755,13 @@
 			return_cmd_error(STR_0007_FLAT_LAND_REQUIRED);
 		}
 
-		/* if station is set, then we have special handling to allow building on top of already existing stations.
-		 * so station points to INVALID_STATION if we can build on any station.
-		 * Or it points to a station if we're only allowed to build on exactly that station. */
+		/* if station is set, then allow building on top of an already
+		 * existing station, either the one in *station if it is not
+		 * INVALID_STATION, or anyone otherwise and store which one
+		 * in *station
+		 * also, only allow building over station tiles of type st */
 		if (station != NULL && IsTileType(tile_cur, MP_STATION)) {
-			if (!IsRailwayStation(tile_cur)) {
+			if (GetStationType(tile_cur) != st) {
 				return ClearTile_Station(tile_cur, DC_AUTO); // get error message
 			} else {
 				StationID st = GetStationIndex(tile_cur);
@@ -1650,6 +1653,37 @@
 	_airport_sections_helistation        // Helistation
 };
 
+/* Checks if an airport can be removed (no aircraft on it or landing) */
+/* Returns cost on success, else an error */
+static CommandCost CanRemoveAirport(Station *st, uint32 flags)
+{
+	TileIndex tile = st->airport_tile;
+
+	const AirportFTAClass *afc = st->Airport();
+	int w = afc->size_x;
+	int h = afc->size_y;
+
+	Vehicle *v;
+	FOR_ALL_VEHICLES(v) {
+		if (!(v->type == VEH_AIRCRAFT && IsNormalAircraft(v))) continue;
+
+		if (v->u.air.targetairport == st->index && v->u.air.state != FLYING)
+			return_cmd_error(STR_A015_AIRCRAFT_IN_THE_WAY);
+	}
+
+	BEGIN_TILE_LOOP(tile_cur, w, h, tile) {
+		if (!EnsureNoVehicleOnGround(tile_cur))
+			return_cmd_error(STR_A015_AIRCRAFT_IN_THE_WAY);
+
+		if (flags & DC_EXEC) {
+			DeleteAnimatedTile(tile_cur);
+			DoClearSquare(tile_cur);
+		}
+	} END_TILE_LOOP(tile_cur, w, h, tile)
+
+	return CommandCost(w * h * _price.remove_airport);
+}
+
 /** Place an Airport.
  * @param tile tile where airport will be built
  * @param flags operation to perform
@@ -1658,8 +1692,6 @@
  */
 CommandCost CmdBuildAirport(TileIndex tile, uint32 flags, uint32 p1, uint32 p2)
 {
-	bool airport_upgrade = true;
-
 	SET_EXPENSES_TYPE(EXPENSES_CONSTRUCTION);
 
 	/* Check if a valid, buildable airport was chosen for construction */
@@ -1668,10 +1700,35 @@
 	if (!(flags & DC_NO_TOWN_RATING) && !CheckIfAuthorityAllows(tile))
 		return CMD_ERROR;
 
+	const AirportFTAClass *afc = GetAirport(p1);
+	int w = afc->size_x;
+	int h = afc->size_y;
+
+	/* Make sure the area below is clear or only overlaps one airport */
+	StationID est = INVALID_STATION;
+	CommandCost cost = CheckFlatLandBelow(tile, w, h, flags & ~DC_EXEC, 0, &est, true, STATION_AIRPORT);
+	if (CmdFailed(cost)) return cost;
+
+	Station *st = NULL;
+
+	if (!_patches.adjacent_stations || !HasBit(p2, 0)) {
+		st = GetStationAround(tile, w, h, est);
+		if (st == CHECK_STATIONS_ERR) return CMD_ERROR;
+	}
+
+	// action to be performed
+	enum {
+		AIRPORT_NEW,      // airport is a new station
+		AIRPORT_ADD,      // add an airport to an existing station
+		AIRPORT_UPGRADE,  // upgrade the airport in a station
+	} action =
+		(est != INVALID_STATION) ? AIRPORT_UPGRADE :
+		(st != NULL) ? AIRPORT_ADD : AIRPORT_NEW;
+
 	Town *t = ClosestTownFromTile(tile, (uint)-1);
 
 	/* Check if local auth refuses a new airport */
-	{
+	if (action != AIRPORT_UPGRADE) {
 		uint num = 0;
 		const Station *st;
 		FOR_ALL_STATIONS(st) {
@@ -1684,21 +1741,6 @@
 		}
 	}
 
-	const AirportFTAClass *afc = GetAirport(p1);
-	int w = afc->size_x;
-	int h = afc->size_y;
-
-	CommandCost ret = CheckFlatLandBelow(tile, w, h, flags, 0, NULL);
-	if (CmdFailed(ret)) return ret;
-	CommandCost cost(ret.GetCost());
-
-	Station *st = NULL;
-
-	if (!_patches.adjacent_stations || !HasBit(p2, 0)) {
-		st = GetStationAround(tile, w, h, INVALID_STATION);
-		if (st == CHECK_STATIONS_ERR) return CMD_ERROR;
-	}
-
 	/* Find a station close to us */
 	if (st == NULL) st = GetClosestStationFromTile(tile);
 
@@ -1711,17 +1753,22 @@
 	 * to test if everything is OK. In this case we need to delete it before return. */
 	AutoPtrT<Station> st_auto_delete;
 
-	if (st != NULL) {
+	if (action != AIRPORT_NEW) {
 		if (st->owner != _current_player)
 			return_cmd_error(STR_3009_TOO_CLOSE_TO_ANOTHER_STATION);
 
 		if (!st->rect.BeforeAddRect(tile, w, h, StationRect::ADD_TEST)) return CMD_ERROR;
 
-		if (st->airport_tile != 0)
-			return_cmd_error(STR_300D_TOO_CLOSE_TO_ANOTHER_AIRPORT);
+		if (action != AIRPORT_UPGRADE) {
+			if (st->airport_tile != 0)
+				return_cmd_error(STR_300D_TOO_CLOSE_TO_ANOTHER_AIRPORT);
+		} else {
+			/* check that old airport can be removed */
+			CommandCost r = CanRemoveAirport(st, flags);
+			if (CmdFailed(r)) return r;
+			cost.AddCost(r);
+		}
 	} else {
-		airport_upgrade = false;
-
 		/* allocate and initialize new station */
 		st = new Station(tile);
 		if (st == NULL) return_cmd_error(STR_3008_TOO_MANY_STATIONS_LOADING);
@@ -1747,6 +1794,20 @@
 	cost.AddCost(_price.build_airport * w * h);
 
 	if (flags & DC_EXEC) {
+		if (action == AIRPORT_UPGRADE) {
+			/* delete old airport if upgrading */
+			TileIndex otile = st->airport_tile;
+			const AirportFTAClass *oafc = st->Airport();
+
+			for (uint i = 0; i < oafc->nof_depots; ++i) {
+				DeleteWindowById(
+					WC_VEHICLE_DEPOT, otile + ToTileIndexDiff(oafc->airport_depots[i])
+				);
+			}
+
+			st->rect.AfterRemoveRect(st, otile, oafc->size_x, oafc->size_y);
+		}
+
 		st->airport_tile = tile;
 		st->AddFacility(FACIL_AIRPORT, tile);
 		st->airport_type = (byte)p1;
@@ -1761,7 +1822,7 @@
 		 * 1. airport is upgraded
 		 * 2. airport is added to existing station (unfortunately unavoideable)
 		 */
-		if (airport_upgrade) UpdateAirplanesOnNewStation(st);
+		if (action != AIRPORT_NEW) UpdateAirplanesOnNewStation(st);
 
 		{
 			const byte *b = _airport_sections[p1];
@@ -1789,38 +1850,21 @@
 	if (_current_player != OWNER_WATER && !CheckOwnership(st->owner))
 		return CMD_ERROR;
 
-	TileIndex tile = st->airport_tile;
-
-	const AirportFTAClass *afc = st->Airport();
-	int w = afc->size_x;
-	int h = afc->size_y;
-
-	CommandCost cost(w * h * _price.remove_airport);
-
-	Vehicle *v;
-	FOR_ALL_VEHICLES(v) {
-		if (!(v->type == VEH_AIRCRAFT && IsNormalAircraft(v))) continue;
-
-		if (v->u.air.targetairport == st->index && v->u.air.state != FLYING) return CMD_ERROR;
-	}
+	CommandCost cost = CanRemoveAirport(st, flags);
+	if (CmdFailed(cost)) return cost;
 
-	BEGIN_TILE_LOOP(tile_cur, w, h, tile) {
-		if (!EnsureNoVehicleOnGround(tile_cur)) return CMD_ERROR;
+	if (flags & DC_EXEC) {
+		TileIndex tile = st->airport_tile;
 
-		if (flags & DC_EXEC) {
-			DeleteAnimatedTile(tile_cur);
-			DoClearSquare(tile_cur);
-		}
-	} END_TILE_LOOP(tile_cur, w, h, tile)
+		const AirportFTAClass *afc = st->Airport();
 
-	if (flags & DC_EXEC) {
 		for (uint i = 0; i < afc->nof_depots; ++i) {
 			DeleteWindowById(
 				WC_VEHICLE_DEPOT, tile + ToTileIndexDiff(afc->airport_depots[i])
 			);
 		}
 
-		st->rect.AfterRemoveRect(st, tile, w, h);
+		st->rect.AfterRemoveRect(st, tile, afc->size_x, afc->size_y);
 
 		st->airport_tile = 0;
 		st->facilities &= ~FACIL_AIRPORT;
Index: src/unmovable_cmd.cpp
===================================================================
--- src/unmovable_cmd.cpp	2007-11-19 22:32:20.000000000 +0100
+++ src/unmovable_cmd.cpp	2007-12-17 20:42:50.000000000 +0100
@@ -76,7 +76,7 @@
 	MarkTileDirtyByTile(tile + TileDiffXY(1, 1));
 }
 
-extern CommandCost CheckFlatLandBelow(TileIndex tile, uint w, uint h, uint flags, uint invalid_dirs, StationID *station, bool check_clear = true);
+extern CommandCost CheckFlatLandBelow(TileIndex tile, uint w, uint h, uint flags, uint invalid_dirs, StationID *station, bool check_clear = true, StationType st = STATION_RAIL);
 
 /** Build or relocate the HQ. This depends if the HQ is already built or not
  * @param tile tile where the HQ will be built or relocated to