Does your area require a garage for every house? Check local rules at city hall.

Where do you live? I live in Florida and with the number of rain and wind storms we have here I would never give up being able to garage a car to keep it protected.

You also need a closet. 14'X22' isn't overly generous for a bedroom, bath and closet, but if the rest of the rooms in the house are on the small side then it wouldn't be out of scale and might not be a bad idea. Do you have room on your lot to build another garage? If your neighborhood has similar homes and some others have done this conversion then it's worth getting a bid on it.

It's done frequently where I live, especially in my neighborhood of small 1950s houses. So many sad rooms with large windows "views" of parked cars. If possible, put the bathroom on the front (driveway) side of the space. If the bedroom has to go on the front side, see if you can install an attractive tiny courtyard, screened by an attractive wood fence, between the driveway and the bedroom wall.

Location dependent, the slab will not be insulated & waterproofed. Garage's in many areas are framed in 2x4's with minimal insulation, and house walls with 2x6's and much denser insulation.