One off built by me and friends, no plans, done from my head as I went along, 2 rooms, each approximately 21 square meters, one an office/cinema room, the second a gym. Inside it is plastered out like a house, led lights, wooden floor, leather sofas, wood-burner, fridge, microwave, TV, surround sound, internet, 3 screens, fully equipped gym with smith machine, dip frame, running machine, benches and free weights. It's fully insulated and double glazed for all year round use. Looks like a big shed/Swiss Chalet from the outside, more like a house on the inside. I use it everyday!!
People who see it a gobsmacked that a DIY builder with no formal training built it!!