[jyl]

No on the one car garage. They suck - I have one - there's never enough room to work on the car. I don't even think I could extract a rear torsion bar in mine.

If the deck is deep enough for a garage, that looks like the logical route. If it is tall enough for a 911 on a lift, that would be gravy. There's lots of PPOT'ers with construction and design backgrounds here who will chime in, sorry that I'm not one of them.

From my inexpert POV, it seems pretty simple, assuming the deck structure is sound. I'm thinking you'd prepare and pour a concrete floor (thick enough where a future lift might go), frame and sheath walls, install garage doors, electrical, siding and paint. If the deck isn't waterproof (e.g. rain drains right through it), then not sure how that's handled.

Anyway, should be less expensive than building a freestanding garage structure, and that space under the deck isn't doing anything else useful.

Oh wait, maybe you'd want the option of someday finishing the basement into living space with a direct daylight from and exit to the backyard, or even extending the basement living space to the under-deck area? Like, an in-law or guest unit, even a rental unit, or family space, etc? In that case, might want to anticipate that future inhabited use, when building the garage.