Cupola walls are extending into room below. How to fix?

[trentclark]

I created a 12' x 12' Cupola in the center of the 120" (10' ceilings) living room below. Roof around it works, windows in the cupola above the roof work, hip roof of cupola works. My problem is that the walls of the cupola extend about a foot into the Living room ceiling below. I've changed dimensions over and over to see if that works, but doesn't. I think it extends the 12 5/8" of the floor thickness ( "L") on the "Structure Specifications" for the "Open Below" room, but am not able to change it.

Does anyone have a clue how to do this?