Just as you wouldn’t use the asphalt parking surface of an open parking lot as the floor of the house, there’s no obligation to use the actual garage structure.

Just as you wouldn’t use the asphalt parking surface of an open parking lot as the floor of the house, there’s no obligation to use the actual garage structure. 25+ years from now, if it’s more economical to use for housing, knock it down and build high-density housing.