We have a summerhouse, bednobs, this will be our second winter with it.
It's actually called a log cabin and is more substantial than many we looked at. It's not insulated and although we thought we may use it through winter, the reality is we haven't. We do use it on very chilly days and have a couple of throws/fleece blankets in there to snuggle under.
My husband laid an armoured electricity cable (cost about £20, easy peasy according to him) to it before the company we bought it from came and erected it. We already had the concrete base as it's where we used to keep our touring caravan before we sold it. It's 4m x 3m and is pretty much east facing due to where it had to go, which is perfect. If it was completely south facing it would get far too hot.
It's used purely as a summerhouse (or gin palace as it's known in the family) although when we were contemplating getting it I did wonder if I'd do any painting out there. The reality is it's too small for both purposes.
It's single glazed, has no gas or water, and is one of the best things we've ever bought :)