if (!IsTileFlat(tile_cur)) {
		if (GetTileZ(tile) == 0 && GetTileZ(tile_cur) == 0) return_cmd_error(STR_ERROR_SITE_UNSUITABLE);
		int z_tile = GetTileMaxZ(tile);
		int z_tile_cur = GetTileMaxZ(tile_cur);
		bool z = Delta(z_tile_cur, z_tile) == 0;
		Slope slope = GetTileSlope(tile_cur);

		/* Mark the tile as already cleared for the terraform command.
		* Do this for all tiles (like trees), not only objects. */
		ClearedObjectArea *coa = FindClearedObject(tile_cur);
		if (coa == NULL) {
			coa = _cleared_object_areas.Append();
			coa->first_tile = tile_cur;
			coa->area = TileArea(tile_cur, 1, 1);
		}
		/* Hide the tile from the terraforming command */
		TileIndex tile_cur_before = coa->first_tile;
		coa->first_tile = INVALID_TILE;
		ret = DoCommand(tile_cur, z ? slope : ComplementSlope(slope), z ? 0 : 1, flags | DC_NO_WATER, CMD_TERRAFORM_LAND);
		coa->first_tile = tile_cur_before;
		if (ret.Failed()) return_cmd_error(STR_ERROR_SITE_UNSUITABLE);
		cost.AddCost(ret);
	}