I have just torn the old attached garage off my house and replaced it with a new garage with a room above and off to the side, plus a rooftop deck. In the course of designing the new structure, I made the very significant discovery that, if the new garage was not actually attached to the existing house, even if it's only an inch away as mine is, the permit and inspection process was very substantially simpler, faster, and cheaper. Not only that, if my old original house falls down for some reason (earthquake??), now I have a second "house" that, due to some steel beam framing construction, stands a much better chance of remaining upright. And, as I mentioned in another post, the new construction has to have gutters and downspouts that feed into rain barrels, for a nice emergency water supply.