Check out ISO containers as a cheap install for a garage. 2nd hand good ones for less than 3k (40ft). Insulation and windows and away you go. Can clad the outside and disguise the shape easily.
"THINK"...............Its a blinking cert Steve, I think you could make a few bob even in your retirement
It was going to cost €8k for an insulated, steel, prefab 6m x 5m garage, plus the concrete pad, plus erection. Similar price applied if it was timber. ‘Hang on’ I thought. ‘I’m no longer cash rich/time poor; I’ll build my own. Plus I’m not so old I can’t sort out my own erection (with a little help) . So I decided to do it on my own. It’ll cost me <€5k all in and will be insulated with 100mm all round and is to my custom design and needs. But you need to have the time and inclination to do it. I can do timber because it suits the rest of the environment.
Great idea. Insulation all round is the key though. The more insulation the less heat needed. 100mm Kingspan under the concrete slab will make a huge difference as well.