diff --git a/OpenRa.Game/BuildingInfluenceMap.cs b/OpenRa.Game/BuildingInfluenceMap.cs
new file mode 100644
index 0000000000..4804af4cfa
--- /dev/null
+++ b/OpenRa.Game/BuildingInfluenceMap.cs
@@ -0,0 +1,42 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using OpenRa.Game.GameRules;
+
+namespace OpenRa.Game
+{
+	class BuildingInfluenceMap
+	{
+		Actor[,] influence = new Actor[128, 128];
+
+		public BuildingInfluenceMap(World world, Player player)
+		{
+			world.ActorAdded +=
+				a => { if (a.traits.Contains<Traits.Building>() && a.Owner == player) AddInfluence(a); };
+			world.ActorRemoved +=
+				a => { if (a.traits.Contains<Traits.Building>() && a.Owner == player) RemoveInfluence(a); };
+		}
+
+		void AddInfluence(Actor a)
+		{
+			foreach (var t in Footprint.Tiles(a.unitInfo.Name, a.Location))
+				if (IsValid(t))
+					influence[t.X, t.Y] = a;
+		}
+
+		void RemoveInfluence(Actor a)
+		{
+			foreach (var t in Footprint.Tiles(a.unitInfo.Name, a.Location))
+				if (IsValid(t))
+					influence[t.X, t.Y] = null;
+		}
+
+		bool IsValid(int2 t)
+		{
+			return !(t.X < 0 || t.Y < 0 || t.X >= 128 || t.Y >= 128);
+		}
+
+		public Actor this[int2 cell] { get { return IsValid(cell) ? influence[cell.X, cell.Y] : null; } }
+	}
+}
diff --git a/OpenRa.Game/Game.cs b/OpenRa.Game/Game.cs
index 7b0f86365e..f3ed92fe07 100644
--- a/OpenRa.Game/Game.cs
+++ b/OpenRa.Game/Game.cs
@@ -23,7 +23,8 @@ namespace OpenRa.Game
 
 		public readonly Dictionary<int, Player> players = new Dictionary<int, Player>();
 
-		public Player LocalPlayer { get { return players[localPlayerIndex]; } }
+		public Player LocalPlayer { get { return players[localPlayerIndex]; } }
+		public BuildingInfluenceMap LocalPlayerBuildings;
 
 		public Game(string mapName, Renderer renderer, int2 clientSize)
 		{
@@ -46,7 +47,9 @@ namespace OpenRa.Game
 
 			pathFinder = new PathFinder(map, terrain.tileSet);
 
-			network = new Network();
+			network = new Network();
+
+			LocalPlayerBuildings = new BuildingInfluenceMap(world, LocalPlayer);
 
 			controller = new Controller(this);		// CAREFUL THERES AN UGLY HIDDEN DEPENDENCY HERE STILL
 			worldRenderer = new WorldRenderer(renderer, this);
@@ -62,11 +65,13 @@ namespace OpenRa.Game
 
 		public bool IsCellBuildable(int2 a)
 		{
+			if (LocalPlayerBuildings[a] != null) return false;
+
 			a += map.Offset;
 
 			return map.IsInMap(a.X, a.Y) &&
-			TerrainCosts.Cost(UnitMovementType.Wheel,
-				terrain.tileSet.GetWalkability(map.MapTiles[a.X, a.Y])) < double.PositiveInfinity;
+				TerrainCosts.Cost(UnitMovementType.Wheel,
+					terrain.tileSet.GetWalkability(map.MapTiles[a.X, a.Y])) < double.PositiveInfinity;
 		}
 
 		public IEnumerable<Actor> FindUnits(float2 a, float2 b)
diff --git a/OpenRa.Game/GameRules/Footprint.cs b/OpenRa.Game/GameRules/Footprint.cs
index 5d6435f4b4..0e3cde75c5 100644
--- a/OpenRa.Game/GameRules/Footprint.cs
+++ b/OpenRa.Game/GameRules/Footprint.cs
@@ -35,5 +35,23 @@ namespace OpenRa.Game.GameRules
 			buildingFootprints = buildings
 				.ToDictionary(a => a.Name, a => a.Pat);
 		}
+
+		public static IEnumerable<int2> Tiles(string name, int2 position)
+		{
+			var footprint = Rules.Footprint.GetFootprint(name);
+			var j = 0;
+
+			foreach (var row in footprint)
+			{
+				var i = 0;
+				foreach (var c in row)
+				{
+					if (c != '_')
+						yield return position + new int2(i, j);
+					++i;
+				}
+				++j;
+			}
+		}
 	}
 }
diff --git a/OpenRa.Game/OpenRa.Game.csproj b/OpenRa.Game/OpenRa.Game.csproj
index 1580c46701..aee4d9b3b5 100644
--- a/OpenRa.Game/OpenRa.Game.csproj
+++ b/OpenRa.Game/OpenRa.Game.csproj
@@ -81,6 +81,7 @@
     <Compile Include="Graphics\LineRenderer.cs" />
     <Compile Include="Graphics\OverlayRenderer.cs" />
     <Compile Include="Graphics\WorldRenderer.cs" />
+    <Compile Include="BuildingInfluenceMap.cs" />
     <Compile Include="IOrderGenerator.cs" />
     <Compile Include="TechTree\Item.cs" />
     <Compile Include="Network\Packet.cs" />
diff --git a/OpenRa.Game/UiOverlay.cs b/OpenRa.Game/UiOverlay.cs
index b7cbf1ebe5..8cfe685eae 100644
--- a/OpenRa.Game/UiOverlay.cs
+++ b/OpenRa.Game/UiOverlay.cs
@@ -1,6 +1,7 @@
 using System.Drawing;
 using OpenRa.Game.Graphics;
 using System;
+using OpenRa.Game.GameRules;
 
 namespace OpenRa.Game
 {
@@ -36,22 +37,8 @@ namespace OpenRa.Game
 			if (!hasOverlay)
 				return;
 
-			var footprint = Rules.Footprint.GetFootprint(name);
-			var j = 0;
-			foreach (var row in footprint)
-			{
-				var i = 0;
-				foreach (var c in row)
-				{
-					if (c != '_')
-						spriteRenderer.DrawSprite(
-							game.IsCellBuildable(position + new int2(i, j)) 
-								? buildOk : buildBlocked,
-							24 * (position + new int2(i, j)), 0);
-					++i;
-				}
-				++j;
-			}
+			foreach (var t in Footprint.Tiles(name,position))
+				spriteRenderer.DrawSprite(game.IsCellBuildable(t) ? buildOk : buildBlocked, 24 * t, 0);
 
 			spriteRenderer.Flush();
 		}
diff --git a/OpenRa.Game/World.cs b/OpenRa.Game/World.cs
index 3fca98f2e3..31d5bd134e 100644
--- a/OpenRa.Game/World.cs
+++ b/OpenRa.Game/World.cs
@@ -13,11 +13,14 @@ namespace OpenRa.Game
 		int lastTime = Environment.TickCount;
 		const int timestep = 40;
 
-		public World(Game game) { this.game = game; }
-
-		public void Add(Actor a) { actors.Add(a); }
-		public void Remove( Actor a ) { actors.Remove( a ); }
-		public void AddFrameEndTask( Action<World> a ) { frameEndActions.Add( a ); }
+		public World(Game game) { this.game = game; }
+
+		public void Add(Actor a) { actors.Add(a); ActorAdded(a); }
+		public void Remove(Actor a) { actors.Remove(a); ActorRemoved(a); }
+		public void AddFrameEndTask( Action<World> a ) { frameEndActions.Add( a ); }
+
+		public event Action<Actor> ActorAdded = _ => { };
+		public event Action<Actor> ActorRemoved = _ => { };
 
 		public void Update()
 		{