Hi,
I just played around for a couple of minutes to build a two floor building:
Ground Floor (UK) / Floor 1 (US)
Floor 1(UK) / Floor 2(US)
Pretty empty 
“keep it simple” they said 
The purpose of this building was to test the floor/ceiling and the stairs, I indeed needed some thin pieces below walls.
The walls were already tested before so no need to add walls inside the building.
Cheers !