How about attics/usable space under the roof ?

Yes please! My sim's house is also based on a house from a series with an attic (and L shaped stairs to make it even more difficult). I have had to improvise and build the third floor inside the roof. The position isn't quite right as I had to avoid the walls peeking out of the roof. The roof also cuts into the walls in build mode which looks awful, but thankfully the roof disappears in live mode so it's playable. It would be nice not to have to pretend though.