Shed on the outside - studio on the inside. 6x2 joists for floor and roof and 4x2 walls. Insulated all over and covered with waterproof breathable membrane that meant I didn't need proper walls for ages! Glass bricks were a pain but essential for that extra wall space.
I had professionals in for the roof and the electrics but everything else was done by myself - with a bit of family help! The whole thing took about 6 weeks... If anybody wants me, I'll be in the shed...