I agree to have all furniture within the floor layer, where all of your walls, windows, doors, etc... exist. But give those items a class (e.g., 'furniture') Even the plumbing fixtures, cabinetry, appliances, stairs are on the floor layer. I then create a separate slab layer, but its Z layer elevation is below that of the floor by the same dimension as that floors thickness. Then I draw a polygon in the slabs shape desired. Then I extrude (up, positive direction) the floors thickness. This way the floor surface is equal to that of the wall layer. You can layer link these 2 layers to see if it worked and you have a true, accurate model.