Both of your answers are right. There are to ways and the better one i will choose. Yes i will try to build a modular building like below and one idea is to say: you are now a window(frame) or door(frame). An other way is to say: you are a normal wall, but a specify number of you are windowframes. The question is how to solve this. A random change from Wall to Windowframe is now integrated but its to random for me.