I have a bathroom and walk-in closet that is more exposed to the elements because it's on top of my garage. It would be hard to route addtional ducts to it from the existing unit, and I guess the existing heat/ac unit in the attic is max'ed out anyway. I would like to put a supplemental unit in a little cubbyhole of an attic space that's adjacent to said bathroom and closet. I do not want to add another compressor outside. What kind of self-contained heat/ac unit can be intstalled in an attic space, and how much electrical power would it require? Can you suggest a specific model name to look at on the internet?
Try looking at a verticle PTAC. Only issue might be you need to feed supply from above and the return must be oposite a wall to the interior. Smallest unit is around 9K BTU. You might be able to put something in the garage from below and duct the unit up into the living space.
You might save a lot of money by improving the insulation between the garage and the cold bathroom/closet. Most bedrooms and other rooms over garages are cold because of inadequate or improperly installed insulation in the garage ceiling. Often an insulation contractor can blow cellulose insulation into the space and fill any voids. Insulating the garage walls and doors can also improve the situation.