Index: src/town_cmd.cpp
===================================================================
--- src/town_cmd.cpp	(revision 27632)
+++ src/town_cmd.cpp	(working copy)
@@ -1044,6 +1044,9 @@
  */
 static bool GrowTownWithRoad(const Town *t, TileIndex tile, RoadBits rcmd)
 {
+	/* don't build road on coasts with only one corner raised */
+	if (IsSlopeWithOneCornerRaised(GetTileSlope(tile)) && IsCoastTile(tile)) return false;
+
 	if (DoCommand(tile, rcmd, t->index, DC_EXEC | DC_AUTO | DC_NO_WATER, CMD_BUILD_ROAD).Succeeded()) {
 		_grow_town_result = GROWTH_SUCCEED;
 		return true;
@@ -1066,6 +1069,8 @@
 	assert(bridge_dir < DIAGDIR_END);
 
 	const Slope slope = GetTileSlope(tile);
+	/* don't build a bridge if the source tile is a coast with only one corner raised */
+	if (IsSlopeWithOneCornerRaised(slope) && IsCoastTile(tile)) return false;
 
 	/* Make sure the direction is compatible with the slope.
 	 * Well we check if the slope has an up bit set in the
@@ -1103,6 +1108,9 @@
 	/* no water tiles in between? */
 	if (bridge_length == 1) return false;
 
+	/* don't build a bridge if the destination tile is a coast with only one corner raised */
+	if (IsSlopeWithOneCornerRaised(GetTileSlope(bridge_tile)) && IsCoastTile(bridge_tile)) return false;
+
 	for (uint8 times = 0; times <= 22; times++) {
 		byte bridge_type = RandomRange(MAX_BRIDGES - 1);
 
@@ -2066,7 +2074,7 @@
 {
 	/* cannot build on these slopes... */
 	Slope slope = GetTileSlope(tile);
-	if ((noslope && slope != SLOPE_FLAT) || IsSteepSlope(slope)) return false;
+	if ((noslope && slope != SLOPE_FLAT) || IsSteepSlope(slope) || (IsSlopeWithOneCornerRaised(slope) && IsCoastTile(tile))) return false;
 
 	/* building under a bridge? */
 	if (IsBridgeAbove(tile)) return false;