@bgoff thank you for the helpful remarks. However - I spent the entire weekend trying to get the model to work - no success. Simplifying the polys scambles up everything.
So I restarted with the original site data, using Pads - what happens ist still not the desired outcome. I am using pads with retaining walls and grade limits with that. There still is topography inside the building sites, but i do not understand why.
Could you maybe have a look at the file and let me know, what I am doing wrong?