diff --git a/isometric-park-fna/Engines/BuildToolEngine.cs b/isometric-park-fna/Engines/BuildToolEngine.cs
--- a/isometric-park-fna/Engines/BuildToolEngine.cs
+++ b/isometric-park-fna/Engines/BuildToolEngine.cs
@@ -25,7 +25,8 @@
            typeof(PreserveComponent),
            typeof(SelectedComponent),
            typeof(ToolComponent),
-           typeof(PointComponent))]
+           typeof(PointComponent),
+           typeof(StructureComponent))]
     [Sends(typeof(ChangeContractStatusMessage),
             typeof(PlaySoundMessage),
             typeof(SingleExpenseMessage))]
@@ -234,6 +235,13 @@
                     occupied.AddRange(entitySquares);
                 }
 
+                foreach (var (entity, point) in ReadEntities<StructureComponent>()
+                        .WhereF((e) => HasComponent<PointComponent>(e))
+                        .Select((e) => (e, GetComponent<PointComponent>(e)))) {
+
+                    occupied.Add(point.Square);
+                }
+
                 foreach (ref readonly var message in ReadMessages<AdjustSelection>()) {
                     foreach (ref readonly   var entity in ReadEntities<SelectedComponent>()) {
                         var selection = GetComponent<SelectedComponent>(entity);
@@ -246,9 +254,10 @@
                             else if(message.Type == AdjustmentType.Complete) {
                                 var point = GetComponent<PointComponent>(entity);
                                 var structure_entity = CreateEntity();
+                                var cell = this.Map.cells[(int)point.Square.X][(int)point.Square.Y];
                                 //Verify square is free:
                                 //
-                                if (!occupied.Contains(point.Square)) {
+                                if (!occupied.Contains(point.Square) && !cell.HasTree && !cell.HasWater) {
 
                                     AddComponent(structure_entity, new PointComponent {Square = point.Square,
                                             Tool = point.Tool} );