I'd watch for a few of things here:
1. Obviously, you lose storage space for cars, junk, etc.
2. Would you have a driveway leading to "nowhere"? (This can look like an obvious cost-cutting garage conversion.)
3. Would it fit with the internal "flow" of the house--or just look like some room "off to the side?"
4. Garages generally have lower floors than the house. Would this have to be built up to house floor level?
So not a real answer per se, but I'd suggest you might be better off building an addition properly in the right place.