diff --git a/src/tunnelbridge_cmd.cpp b/src/tunnelbridge_cmd.cpp
index f7269fd7c7..8ca01caac6 100644
--- a/src/tunnelbridge_cmd.cpp
+++ b/src/tunnelbridge_cmd.cpp
@@ -328,9 +328,6 @@ CommandCost CmdBuildBridge(DoCommandFlag flags, TileIndex tile_end, TileIndex ti
 	auto [tileh_end, z_end] = GetTileSlopeZ(tile_end);
 	bool pbs_reservation = false;
 
-	CommandCost terraform_cost_north = CheckBridgeSlope(BRIDGE_PIECE_NORTH, direction, tileh_start, z_start);
-	CommandCost terraform_cost_south = CheckBridgeSlope(BRIDGE_PIECE_SOUTH, direction, tileh_end,   z_end);
-
 	/* Aqueducts can't be built of flat land. */
 	if (transport_type == TRANSPORT_WATER && (tileh_start == SLOPE_FLAT || tileh_end == SLOPE_FLAT)) return CommandCost(STR_ERROR_LAND_SLOPED_IN_WRONG_DIRECTION);
 	if (z_start != z_end) return CommandCost(STR_ERROR_BRIDGEHEADS_NOT_SAME_HEIGHT);
@@ -408,24 +405,71 @@ CommandCost CmdBuildBridge(DoCommandFlag flags, TileIndex tile_end, TileIndex ti
 	} else {
 		/* Build a new bridge. */
 
-		bool allow_on_slopes = (_settings_game.construction.build_on_slopes && transport_type != TRANSPORT_WATER);
-
 		/* Try and clear the start landscape */
 		CommandCost ret = Command<CMD_LANDSCAPE_CLEAR>::Do(flags, tile_start);
 		if (ret.Failed()) return ret;
 		cost = ret;
 
-		if (terraform_cost_north.Failed() || (terraform_cost_north.GetCost() != 0 && !allow_on_slopes)) return CommandCost(STR_ERROR_LAND_SLOPED_IN_WRONG_DIRECTION);
-		cost.AddCost(terraform_cost_north);
-
 		/* Try and clear the end landscape */
 		ret = Command<CMD_LANDSCAPE_CLEAR>::Do(flags, tile_end);
 		if (ret.Failed()) return ret;
 		cost.AddCost(ret);
 
-		/* false - end tile slope check */
-		if (terraform_cost_south.Failed() || (terraform_cost_south.GetCost() != 0 && !allow_on_slopes)) return CommandCost(STR_ERROR_LAND_SLOPED_IN_WRONG_DIRECTION);
-		cost.AddCost(terraform_cost_south);
+		/* for aqueducts, slope of end tile must be complementary to the slope of the start tile */
+		if (transport_type == TRANSPORT_WATER && tileh_end != ComplementSlope(tileh_start)) {
+			/* Check if the tile to be terraformed is the start or end tile */
+			TileIndex terraformTile;
+			Slope terraformTileh;
+			if (tileh_start == SLOPE_N || tileh_start == SLOPE_S || tileh_start == SLOPE_W || tileh_start == SLOPE_E) {
+				terraformTile = tile_start;
+				terraformTileh = tileh_start ^ ComplementSlope(tileh_end);
+			} else {
+				terraformTile = tile_end;
+				terraformTileh = tileh_end ^ ComplementSlope(tileh_start);
+			}
+
+			/* Mark the tile as already cleared for the terraform command.
+			 * Do this for all tiles (like trees), not only objects. */
+			ClearedObjectArea *coa = FindClearedObject(terraformTile);
+			if (coa == nullptr) {
+				coa = &_cleared_object_areas.emplace_back(ClearedObjectArea{ terraformTile, TileArea(terraformTile, 1, 1) });
+			}
+
+			/* Hide the tile from the terraforming command */
+			TileIndex old_first_tile = coa->first_tile;
+			coa->first_tile = INVALID_TILE;
+
+			/* CMD_TERRAFORM_LAND may append further items to _cleared_object_areas,
+			 * however it will never erase or re-order existing items.
+			 * _cleared_object_areas is a value-type self-resizing vector, therefore appending items
+			 * may result in a backing-store re-allocation, which would invalidate the coa pointer.
+			 * The index of the coa pointer into the _cleared_object_areas vector remains valid,
+			 * and can be used safely after the CMD_TERRAFORM_LAND operation.
+			 * Deliberately clear the coa pointer to avoid leaving dangling pointers which could
+			 * inadvertently be dereferenced.
+			 */
+			ClearedObjectArea *begin = _cleared_object_areas.data();
+			assert(coa >= begin && coa < begin + _cleared_object_areas.size());
+			size_t coa_index = coa - begin;
+			assert(coa_index < UINT_MAX); // more than 2**32 cleared areas would be a bug in itself
+			coa = nullptr;
+
+			ret = std::get<0>(Command<CMD_TERRAFORM_LAND>::Do(DoCommandFlag(flags & ~DC_NO_WATER), terraformTile, terraformTileh, true));
+			_cleared_object_areas[(uint)coa_index].first_tile = old_first_tile;
+			if (ret.Failed()) return CommandCost(STR_ERROR_LAND_SLOPED_IN_WRONG_DIRECTION);
+			cost.AddCost(ret);
+		}
+
+		if (transport_type != TRANSPORT_WATER){
+			CommandCost terraform_cost_north = CheckBridgeSlope(BRIDGE_PIECE_NORTH, direction, tileh_start, z_start);
+			if (terraform_cost_north.Failed() || (terraform_cost_north.GetCost() != 0 && !_settings_game.construction.build_on_slopes)) return CommandCost(STR_ERROR_LAND_SLOPED_IN_WRONG_DIRECTION);
+			cost.AddCost(terraform_cost_north);
+
+			CommandCost terraform_cost_south = CheckBridgeSlope(BRIDGE_PIECE_SOUTH, direction, tileh_end, z_end);
+			/* false - end tile slope check */
+			if (terraform_cost_south.Failed() || (terraform_cost_south.GetCost() != 0 && !_settings_game.construction.build_on_slopes)) return CommandCost(STR_ERROR_LAND_SLOPED_IN_WRONG_DIRECTION);
+			cost.AddCost(terraform_cost_south);
+		}
 
 		const TileIndex heads[] = {tile_start, tile_end};
 		for (int i = 0; i < 2; i++) {