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I'd measure the largest vehicles you intend to have in there, and if you intend to work on/around them leave a good amount of room... 24-28' deep is probably better to get working room. Or, go longer and enter the gable end of the garage. I'm about certain you cannot enter the sleeping quarters directly from the garage, I believe chapter 3 of the IRC. Through the bath, or if it's deeper, through a hall to bath and then living quarters would probably work. 30 minute or higher doors seperating, and watch the penetrations between uses. An I beam across the walls with columns down to good footings, with a trolley for engine pulling would not be a bad thing :). 9' walls or higher for standard rollup door hardware.
If I lived anywhere there is appreciable amount of snow and we do. I sure would chance my plan to a gable end door rather than shovel snow sliding off the roof and try and scurry off to an appointment or work.
Second... Building something that encumbering could very well and very soon prove to be a waste of money. Remembering goals of today are tomorrows tee shirts hanging in the closet. That said I would look at apartments over garages. John Raabe who hosts this site has a pay site titled http://www.planhelp.com/ where he had one that was very efficient. I gleaned a couple when we were looking that were great. (They never got built.) Someday you may wish to sell this and find something larger. I think it would be a real problem on the market.
You both have some excellent point's I need to take in to consideration. Thank You.
......EDIT......
Mailed off my order for the 20x30 single story with material list and saltbox plans and will be joining the planhelp site soon, decided to keep reviewing my options in tell I have gone over the plans and not tie myself down to one layout or another just yet...