Before we ever considered building a guest house, I bought a version of Punch to model interior changes at our old house. It was cheap IIRC. I don't have the CD to look at but it is probably their $50 version:
Punch! Home & Landscape Design 17.5 | Punch Software | Official Site
If you want to get fancy they have more powerful versions.
Home Design | Punch Software | Official Site
The program is kinda like a watered down CAD program with pre-drawn features that you pick and drop. You can the render it in 3D and do a walk through. It'll also give you the material list so you can price material.
The architect who took my design and turned it into a permit drawing was a MAJOR disappointment. He gave me back 4 sheets and 1/2 of them was typical building details that was scanned and layed on the drawing. When it came time to build, the builder and I just went off my drawings.
But the architect's drawing were good enough for permitting. I live in a rural area and the building inspector knows I am an engineer with design and construction experience so it probably helped.
Talk to your building and zoning department. They will help you through the permitting process and recommend an architect who will make permitting easier.
I recommend you play with the house design (which ever program you use). Design it, print the design, set it on the dinner table and make fly by reviews, redesign and repeat until you are completely out of ideas. You'll make changes during construction, but the dinner table fly by reviews will minimize the cost of construction changes.