How to stop placing a building on an obstacle

Question

I am trying to make a city builder game and my problem is that I can't figure out a good way how to make a script that would not let a building to be placed if there is another building placed in that position or just an obstacle. I tried making a script with triggers to stop buildings from being built, but that didn't seem to work as I expected, it gave me a quite big fps drop whenever I was moving my mouse over the buildings with a buildable building selected and sometimes I could even build overlapping the other buildings . Here are my scripts. Any help would be appreciated.

Building placement script

 #pragma strict
 
 private var hit : RaycastHit;
 var gridSize : float = 1.0;
 private var canBuild = false;
 var BuildableBuilding : GameObject;
 
 function Update () {
 
     var ray : Ray;
     ray = Camera.main.ScreenPointToRay(Input.mousePosition);
     if (Physics.Raycast(ray, hit, 100.0f))
     {
         if (hit.collider.name=="Ground") 
         {
                 GameObject.FindWithTag("Building").transform.position = hit.point + Vector3(0,0.5,0); 
                  GameObject.FindWithTag("Building").transform.position.x = Mathf.Round(GameObject.FindWithTag("Building").transform.position.x / gridSize) * gridSize;
                 GameObject.FindWithTag("Building").transform.position.z = Mathf.Round(GameObject.FindWithTag("Building").transform.position.z / gridSize) * gridSize;
         }
     }
 }
 
 function OnGUI() {
         if (GUI.Button(Rect(10,70,50,30),"Build"))
         {
             Debug.Log("Choose a place to build");
             canBuild = true;
             Instantiate(BuildableBuilding, hit.point, Quaternion.identity);
             canBuild = false;
         }
     }

FinishBuilding script

 #pragma strict
 
 var newMat : Material;
 var oldMat : Material;
 var finishedBuilding : GameObject;
 
 private var canBuild = true;
 
 function Update () {
         if (Input.GetButton ("Shoot") && canBuild == true)
         {
         Instantiate(finishedBuilding, transform.position, transform.rotation);
         Destroy(gameObject);
         }
 }
 
 function OnTriggerStay (other : Collider) {
     if(other.gameObject.tag=="BuildingDone")
         {
         canBuild = false;
         renderer.material = newMat;
         print("new material");
         }
     }
 
 function OnTriggerExit (other : Collider) {
     if(other.gameObject.tag=="BuildingDone")
         {
         canBuild = true;
         renderer.material = oldMat;
         print("old material");
         }
     }

Answer 1

Alright, I think I solved my problem. The reason why I could sometimes build on the other buildings is because it didn't know how many triggers it had to exit. For example if I placed 2 buildings near each other and tried to place a third one on them I could do that because the trigger would be exiting one of my buildings and not both so all I had to do is count how many triggers does it enter and leave. Here is the final code.

 #pragma strict
 
 var colliderCount : int = 0;
 var newMat : Material;
 var oldMat : Material;
 var finishedBuilding : GameObject;
 
 function Update () {
         if (Input.GetButton ("Shoot") && colliderCount == 0)
         {
         Instantiate(finishedBuilding, transform.position, transform.rotation);
         Destroy(gameObject);
         }
 }
 
 function OnTriggerEnter (other : Collider) {
     if(other.gameObject.tag=="BuildingDone")
         {
         colliderCount++;
         renderer.material = newMat;
         print("new material");
         }
     }
 
 function OnTriggerExit (other : Collider) {
     if(other.gameObject.tag=="BuildingDone")
         {
         colliderCount--;
             if(colliderCount == 0)
             {
             renderer.material = oldMat;
             print("old material");
             }
         }
     }

How to stop placing a building on an obstacle

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