I guess you don't have a usable basement?
My house is a hair over 1000 sq. ft with a full basement, subtract out the mechanical stuff down there and I've probably got about 900 sq ft. for pins, whatever.
Can you make part of the garage 'part of the house'? i.e. put up a wall and extend your heating/cooling out to it? (2 car garage to 1 car).