--- a/src/autoslope.h
+++ b/src/autoslope.h
@@ -33,7 +33,7 @@
  */
 static inline bool AutoslopeCheckForEntranceEdge(TileIndex tile, uint z_new, Slope tileh_new, DiagDirection entrance)
 {
-	if (IsSteepSlope(tileh_new) || (GetTileMaxZ(tile) != z_new + GetSlopeMaxZ(tileh_new))) return false;
+	if (GetTileMaxZ(tile) != z_new + GetSlopeMaxZ(tileh_new)) return false;
 	return ((tileh_new == SLOPE_FLAT) || CanBuildDepotByTileh(entrance, tileh_new));
 }
 

--- a/src/depot_func.h
+++ b/src/depot_func.h
@@ -26,19 +26,14 @@
  * @param direction The direction in which the depot's exit points
  * @param tileh The slope of the tile in question
  * @return true if the construction is possible
-
- * This is checked by the ugly 0x4C >> direction magic, which does the following:
- * 0x4C is 0100 1100 and tileh has only bits 0..3 set (steep tiles are ruled out)
- * So: for direction (only the significant bits are shown)<p>
- * 00 (exit towards NE) we need either bit 2 or 3 set in tileh: 0x4C >> 0 = 1100<p>
- * 01 (exit towards SE) we need either bit 1 or 2 set in tileh: 0x4C >> 1 = 0110<p>
- * 02 (exit towards SW) we need either bit 0 or 1 set in tileh: 0x4C >> 2 = 0011<p>
- * 03 (exit towards NW) we need either bit 0 or 4 set in tileh: 0x4C >> 3 = 1001<p>
- * So ((0x4C >> direction) & tileh) determines whether the depot can be built on the current tileh
  */
 static inline bool CanBuildDepotByTileh(DiagDirection direction, Slope tileh)
 {
-	return ((0x4C >> direction) & tileh) != 0;
+	assert(tileh != SLOPE_FLAT);
+	Slope entrance_corners = InclinedSlope(direction);
+	/* For steep slopes both entrance corners must be raised (i.e. neither of them is the lowest corner),
+	 * For non-steep slopes at least one corner must be raised. */
+	return IsSteepSlope(tileh) ? (tileh & entrance_corners) == entrance_corners : (tileh & entrance_corners) != 0;
 }
 
 #endif /* DEPOT_FUNC_H */

--- a/src/object_cmd.cpp
+++ b/src/object_cmd.cpp
@@ -168,7 +168,7 @@
 	}
 }
 
-extern CommandCost CheckBuildableTile(TileIndex tile, uint invalid_dirs, int &allowed_z, bool check_bridge);
+extern CommandCost CheckBuildableTile(TileIndex tile, uint invalid_dirs, int &allowed_z, bool allow_steep, bool check_bridge);
 static CommandCost ClearTile_Object(TileIndex tile, DoCommandFlag flags);
 
 /**
@@ -238,7 +238,7 @@
 			}
 
 			if (callback == CALLBACK_FAILED) {
-				cost.AddCost(CheckBuildableTile(t, 0, allowed_z, false));
+				cost.AddCost(CheckBuildableTile(t, 0, allowed_z, false, false));
 			} else if (callback != 0) {
 				/* The meaning of bit 10 is inverted in the result of this callback. */
 				return GetErrorMessageFromLocationCallbackResult(ToggleBit(callback, 10), spec->grf_prop.grffile->grfid, STR_ERROR_LAND_SLOPED_IN_WRONG_DIRECTION);

--- a/src/rail_cmd.cpp
+++ b/src/rail_cmd.cpp
@@ -874,7 +874,6 @@
 
 	if (tileh != SLOPE_FLAT && (
 				!_settings_game.construction.build_on_slopes ||
-				IsSteepSlope(tileh) ||
 				!CanBuildDepotByTileh(dir, tileh)
 			)) {
 		return_cmd_error(STR_ERROR_FLAT_LAND_REQUIRED);
@@ -2338,15 +2337,15 @@
 
 static uint GetSlopeZ_Track(TileIndex tile, uint x, uint y)
 {
-	uint z;
-	Slope tileh = GetTileSlope(tile, &z);
-
-	if (tileh == SLOPE_FLAT) return z;
 	if (IsPlainRail(tile)) {
+		uint z;
+		Slope tileh = GetTileSlope(tile, &z);
+		if (tileh == SLOPE_FLAT) return z;
+
 		z += ApplyFoundationToSlope(GetRailFoundation(tileh, GetTrackBits(tile)), &tileh);
 		return z + GetPartialZ(x & 0xF, y & 0xF, tileh);
 	} else {
-		return z + TILE_HEIGHT;
+		return GetTileMaxZ(tile);
 	}
 }
 

--- a/src/road_cmd.cpp
+++ b/src/road_cmd.cpp
@@ -918,7 +918,6 @@
 	Slope tileh = GetTileSlope(tile, NULL);
 	if (tileh != SLOPE_FLAT && (
 				!_settings_game.construction.build_on_slopes ||
-				IsSteepSlope(tileh) ||
 				!CanBuildDepotByTileh(dir, tileh)
 			)) {
 		return_cmd_error(STR_ERROR_FLAT_LAND_REQUIRED);
@@ -1343,16 +1342,17 @@
 
 static uint GetSlopeZ_Road(TileIndex tile, uint x, uint y)
 {
-	uint z;
-	Slope tileh = GetTileSlope(tile, &z);
 
-	if (tileh == SLOPE_FLAT) return z;
 	if (IsNormalRoad(tile)) {
+		uint z;
+		Slope tileh = GetTileSlope(tile, &z);
+		if (tileh == SLOPE_FLAT) return z;
+
 		Foundation f = GetRoadFoundation(tileh, GetAllRoadBits(tile));
 		z += ApplyFoundationToSlope(f, &tileh);
 		return z + GetPartialZ(x & 0xF, y & 0xF, tileh);
 	} else {
-		return z + TILE_HEIGHT;
+		return GetTileMaxZ(tile);
 	}
 }
 

--- a/src/station_cmd.cpp
+++ b/src/station_cmd.cpp
@@ -672,10 +672,11 @@
  * @param tile TileIndex to check.
  * @param invalid_dirs Prohibited directions for slopes (set of #DiagDirection).
  * @param allowed_z Height allowed for the tile. If allowed_z is negative, it will be set to the height of this tile.
+ * @param allow_steep Whether steep slopes are allowed.
  * @param check_bridge Check for the existance of a bridge.
  * @return The cost in case of success, or an error code if it failed.
  */
-CommandCost CheckBuildableTile(TileIndex tile, uint invalid_dirs, int &allowed_z, bool check_bridge = true)
+CommandCost CheckBuildableTile(TileIndex tile, uint invalid_dirs, int &allowed_z, bool allow_steep, bool check_bridge = true)
 {
 	if (check_bridge && MayHaveBridgeAbove(tile) && IsBridgeAbove(tile)) {
 		return_cmd_error(STR_ERROR_MUST_DEMOLISH_BRIDGE_FIRST);
@@ -691,23 +692,21 @@
 	 *   1) The tile is "steep" (i.e. stretches two height levels).
 	 *   2) The tile is non-flat and the build_on_slopes switch is disabled.
 	 */
-	if (IsSteepSlope(tileh) ||
+	if ((!allow_steep && IsSteepSlope(tileh)) ||
 			((!_settings_game.construction.build_on_slopes) && tileh != SLOPE_FLAT)) {
 		return_cmd_error(STR_ERROR_FLAT_LAND_REQUIRED);
 	}
 
 	CommandCost cost(EXPENSES_CONSTRUCTION);
-	int flat_z = z;
+	int flat_z = z + GetSlopeMaxZ(tileh);
 	if (tileh != SLOPE_FLAT) {
 		/* Forbid building if the tile faces a slope in a invalid direction. */
-		if ((HasBit(invalid_dirs, DIAGDIR_NE) && !(tileh & SLOPE_NE)) ||
-		    (HasBit(invalid_dirs, DIAGDIR_SE) && !(tileh & SLOPE_SE)) ||
-		    (HasBit(invalid_dirs, DIAGDIR_SW) && !(tileh & SLOPE_SW)) ||
-		    (HasBit(invalid_dirs, DIAGDIR_NW) && !(tileh & SLOPE_NW))) {
-			return_cmd_error(STR_ERROR_FLAT_LAND_REQUIRED);
+		for (DiagDirection dir = DIAGDIR_BEGIN; dir != DIAGDIR_END; dir++) {
+			if (HasBit(invalid_dirs, dir) && !CanBuildDepotByTileh(dir, tileh)) {
+				return_cmd_error(STR_ERROR_FLAT_LAND_REQUIRED);
+			}
 		}
 		cost.AddCost(_price[PR_BUILD_FOUNDATION]);
-		flat_z += TILE_HEIGHT;
 	}
 
 	/* The level of this tile must be equal to allowed_z. */
@@ -733,7 +732,7 @@
 	int allowed_z = -1;
 
 	TILE_AREA_LOOP(tile_cur, tile_area) {
-		CommandCost ret = CheckBuildableTile(tile_cur, 0, allowed_z);
+		CommandCost ret = CheckBuildableTile(tile_cur, 0, allowed_z, true);
 		if (ret.Failed()) return ret;
 		cost.AddCost(ret);
 
@@ -769,7 +768,7 @@
 	bool slope_cb = statspec != NULL && HasBit(statspec->callback_mask, CBM_STATION_SLOPE_CHECK);
 
 	TILE_AREA_LOOP(tile_cur, tile_area) {
-		CommandCost ret = CheckBuildableTile(tile_cur, invalid_dirs, allowed_z);
+		CommandCost ret = CheckBuildableTile(tile_cur, invalid_dirs, allowed_z, false);
 		if (ret.Failed()) return ret;
 		cost.AddCost(ret);
 
@@ -851,7 +850,7 @@
 	int allowed_z = -1;
 
 	TILE_AREA_LOOP(cur_tile, tile_area) {
-		CommandCost ret = CheckBuildableTile(cur_tile, invalid_dirs, allowed_z);
+		CommandCost ret = CheckBuildableTile(cur_tile, invalid_dirs, allowed_z, !is_drive_through);
 		if (ret.Failed()) return ret;
 		cost.AddCost(ret);
 
@@ -3586,7 +3585,7 @@
 		/* TODO: If you implement newgrf callback 149 'land slope check', you have to decide what to do with it here.
 		 *       TTDP does not call it.
 		 */
-		if (!IsSteepSlope(tileh_new) && (GetTileMaxZ(tile) == z_new + GetSlopeMaxZ(tileh_new))) {
+		if (GetTileMaxZ(tile) == z_new + GetSlopeMaxZ(tileh_new)) {
 			switch (GetStationType(tile)) {
 				case STATION_WAYPOINT:
 				case STATION_RAIL: {