Thanks for the comments and help guys.

This said, I'm just giving up on this. Going 3D from a map "designed" in 2D is not the way it works, clearly. For example, I realize that I should have a lot more inner yards and small streets at this scale, to avoid big housing blocks with no lights at all in the middlle...
Another thing : the shape of my house blocks : almost none with 90° corners, making it really difficult to get some "normal" construction pattern.
Well, the buildings I already designed will be used anyway. I can do smaller towns/villages correctly with those. A good lesson anyway!