It isn't so much buildings as logistics of laying out a room.

[Alucard at this point scoots over, so Percy can see the full of the work. There's tracing paper put over the current draft of plans, with the outline of the intended new building space but no interior work.]

The key is balancing the placement of the stairs and the kitchen so that nothing is too awkward to move around in. I haven't found something I think works.