Back again.
Please pay attention to next points:
Buildings are in general rather rectangular. just 4, 6 or 8 nodes are sufficient.
Each rectangular building need to be squared. !!  Q key when still selected
Nodes of buildings may not connect to other buildings or features.
The shadow of a building is not a part of the building. 
this document might be helpful to improve your mapping skills
https://docs.google.com/document/d/1Swwz-ZwM3_YOstqstsmCg1-lNAPbnMLpi1Al-h-2UOQ/edit
