DBUser I see, those are of a different nature. Your going to have to learn how to model a staircase and/or your own custom builds.
Or do you mean ( by building exteriors ) the segments for buildings?
But the floors, that's just a plain old "re-texture" of an existing floor. FirmaV2 can do that easily with your own textures.
Firma can do floors,walls ( inside or out) all your doing is changing the texture of an existing one.
There are a whole lot of tutorials on Youtube for fpsc, I have a bunch myself.
** WHERE'S MY eXplosys ? **