gg67
I would have 4 layers. The lowest would have levels 1 & 2, the next up would have levels 3 & 4, the next layer would have the top floor level and the top layer would have the roof.
Walls - you could start the walls from the bottom slab, but (1) that would mean creating look-alike 2d walls on the upper layers (for printing) and (2) it is difficult to have windows above one another in the same wall.
I would have walls on the layers I have described, and you can adjust the bottom and tops of the walls, and the vertical position of windows and doors, to fit the floors if you want.