I went the other route when I built the house. As the area for the garage was already excavated to put the footings in, I got an Engineer to do up some drawings for a structural slab, and now have a room under the garage, accessible from the basement. Can still park vehicles in the garage (I don't as it's more of a workshop, but I have on occasion) and gained about 400 sq ft of room in the house. The size of the garage wasn't optimized for vehicles, but to ensure that I could put a 6 X 12 snooker table in the room underneath, with no need for shorty cues. Cost about $4000 for those 400 sq ft. Heat the garage with a 1500 w electric heater when I need to.