Saltbox single-story garage

[degrandpre]

Help!

Using Home Designer Architectural 2014, I'm trying to build a single story saltbox garage with a design similar to the attached picture. I've read the help file, I've read all links and advice previously provided to no avail. The only suggestions that have sort of worked involve building a second floor which I'd like to avoid as it will render the materials list useless.

Here are my key dimensions:

Length - 24'
Width - 16'
Front wall height - 10'
Rear wall height - 6'
Front roof pitch - 12:12
Rear roof pitch - 8:12

I'm out of ideas on how to tackle this...