I just did this for converting my attic. You'll need to add a second floor. It was a weird concept to me but it's how the software works.
Posted February 5, 2015
For adding attic type rooms under an existing roof system, the key is to accurately emulate the existing home. Then you "save-as" a copy of the "as-built" accurate model, then add a "blank Second floor" without rebuilding roofs. Then when you draw the walls for living space in what was the attic, you end up with what possible, living space is then available. It should be done carefully, checking the virtual model against the existing house with lots of dimensions to make sure that your copy is faithful to the original.
DJP
there are several articles on this. I found the easiest thing is to add a second floor and start with blank and not create from floor plan. That's more of a story and a half which is what I just did.
thebeedubya's post in Bonus area over garage was marked as the answer