In '3D View' in Menu it already has the "Display all levels" checked.
There should be a floor, so I shouldn't be able to see the bottom of the stairs. I shouldn't be able to see the doorway, because it is below the floor. I looked in one of the demos. You have a 3-storey house demo. There are floors in it. You can't see through them. But when I started adding walls to the 2nd floor in mine, the floor/ceiling disappeared.
