Summer house ideas

We have a little summer house at the moment - it's kind of an odd shape with a permanent opening. It's a bit ramshackle to be honest, and Jay has his heart set on a new summer house with sliding or opening glass doors along most of the length of the 'rectangle' facing the house, and thus also the afternoon/evening sun.

We will draw electric into it, probably also have the existing solar panels set up on the roof, which needs to slope backwards towards the back of the garden, if that makes sense. I'd quite a little sink of some kind put in, but not sure about rules around the waste.

Have you built one? What tips and tricks do you have up your sleeves? What would you do, if you built one again? What's on your wishlist?