Take a look:
I have to separate slabs, one for construction and one for flooring. This way you can have different floorings in various rooms with the same construction slab. Then, I put doors - but here is the only drawback, I have to set different level for doors, as normally they would be put at the top of construction. Or you can do it any other way, just moving doors upwards, for example. The next things are floorings, without doorsteps, this is the final move. Of course, you could put them at any time, but every change would cost you additional work. So, at doorsteps I add rectangles and join them with the flooring I want. Finally I extend wall downwards (you can’t see this at the picture, but I choose the “bottom” option). That’s all, I have walls standing on a slab, with floorings on all surfaces.
edit: this way you can also decide what layer of a wall starts on whal level, you can offset bottoms as you wish - for example, you can have final coating starting just above flooring