How to make Trees disappear with building placement

You could use OnCollisionEnter(). If you detect that a tree is colliding with a building then use Destroy() to destroy the tree game object.

The other option would be to create a method that ray casts out by a radius from the buildings center. If it hits a tree Destroy() it then rotate the angle by a small degree. Ray cast again until you have ray cast all 360 degrees.

If your trees have colliders, then you could simply do an OnCollisionEnter with your houses, and if the collided object is a tree, disable or delete that tree.

If your trees aren't going to have colliders, well, you could iterate through all your scene objects and calculate if the tree falls within the bounds of your house, but that wouldn't be good for in-game performance. So I'd definitely go with the collider method.

A script, collider and rigidbody with isKinematic = true on every building, and in the script

 void OnCollisionEnter(Collision col)
 {
     if(col.collider.gameObject.CompareTag("Tree")
     {
         Destroy(col.collider.gameObject);
     }
 }

Your trees should have a tag of "Tree" for this method to work. Of you can compare the gameObject.name.Contains("Tree") if you know all your trees will have "Tree" in their name.