Do you know how to build a closet ? 2x4's and all that ? That would be a stand alone. It will need an outside door to seal it. If it's on concrete in a garage it can all be 2x4 on 16 inch centers.Insulation fits that. You can put ply over the insulation but heavy plastic is good too. Inside put 3/4 inch floor advantex in water resistant. and then 1/4 inch luan plywood on walls and ceiling. Cut hole for vents and A/C, install door. Wrap the inside with mylar. Hook up some lights and fans with electro. You can put a small A/C in with the exhaust in the garage and not hurt a thing.

Do you want just one room or two ? One is easy.

I'll start building tomarrow. LOL