Another idea since he's determined on the garage, and its still being built, is to add a small room in the garage. It could be in the corner connecting to the main house with its access door inside the house but closed off to the garage. He could even use 3' or 4' of an entire wall thus building a false room that no one knows about.
Not that expensive and you could run an AC/heat vent to it for climate control.
Not that expensive and you could run an AC/heat vent to it for climate control.