How do I make accessible buildings in Unity 4

Im making a short experimental horror game and I understand allot of things about modelling and stuff but how do I make a good building that you can enter and exit?

using blender and unity cube meshes dosent seem right or if it is right can someone tell me how to do it?

I found the same problem while creating my games, i created this script, apply it to a invisible object and set the box collider to trigger, also when you walk out of the box the GUI disappears here is my code:

var showMessage : boolean;

function OnGUI(){
   	// Make a background box
	GUI.Box (Rect (158,10,100,90), "Enter house");

	// Make the first button. If it is pressed, Application.Loadlevel (1) will be executed
	if (GUI.Button (Rect (168,40,80,20), "enter")) {
		Application.LoadLevel (1);

	// Make the second button.
	GUI.Label (Rect (168,70,80,20), "   my house"); 

//makes it so when you walk in it shows the GUI
function OnTriggerEnter(){
   showMessage = true;
//makes it so when you walk out of the object the GUI disapears 
function OnTriggerExit(){
   showMessage = false;