ok, it's house_h
attempts to fix it failed so far, will look at it later
The problem is the delta between the ceiling over the first cell of the stairway and the floor height of the 2nd cell.
On top of that delta, we also get a little bonus from the height at the *border(+/- 2)* between those cells.
Moving the stairs by two units was an attempt to get the bonus without changing the *look* too much.
If you continue to fail, plz send me the .map file of your attempt.
I also noticed that in the upper part of the staircase, there seem to be some thin brushes with a misplaced carpet texture.