complex addition walls

[Gene]

I'm trying to draw a somewhat complex addition: a 12' overall wall height, +21" level in the bedroom and closet, 12' ceiling height in the entry and hall, and a loft over the dining room addition, partly above a diagonal wall.

I can almost get what I want with a normal 96" first floor with the diagonal wall, and a second floor representing the loft space, but when I try to set the 'second floor' to be 48", the best I can get is it pushing the walls down into the windows and other openings of the first floor.

The linked picture shows the walls at normal height:
http://www.designsinlight.com/images/house2.jpg

How can I set the second floor walls to 48" high without pushing them down into the first floor? Or is there a better way to get the effect that I want?

thanks,
gene