I have a metal 40 x 40 x 16 RV storage and if I was going to do it again here is what I would do. Have insulation sprayed on the ceiling and sides, figure out where you might add a lift one day and make sure the slab is poured thicker there and also might want to add tubing in the floor for under floor heat, the tubing is really cheap to do before the slab is poured. My biggest problem right now with mine is all the heat escaping through the roof in the winter. A 200k space heater barely keeps up.
