The ceiling above the kitchen is pretty much two 16x16 baseplates across a gap 32x16, which is the size of the room. I never actually built the house on a proper baseplate-sized platform, mainly because I had already put down floor tiles, and partly because it's fun to make floating floors. I believe the whole thing is now stuck together by the bookcases, one of which is attached to the baseplate near the stairs, which is in turn supported by the stairs themselves.
Basically the entire right-hand wing of the house makes no sense and is held together by bookcases.