Although some garden office suppliers do provide a made-to-measure service, if you've got the money then you could also think about using an architect, especially if you're after something a little special. You can get a list of local architects from Architect Search or Royal Institute of British Architects. Two examples of what can be done are pictured. The more traditional shed, a cabin for a north London writer who wanted a place to work surrounded by nature, was designed by Sarah Wigglesworth. Designed to maximise light, it has various spots for sitting, thinking, writing and reading and also - which is going on my wishlist - a daybed built high into the back wall. It also has a small kitchen and a grass roof.
Or there's the more futuristic example, the Garden Studio at Odstock Manor in Wiltshire by Klaentschi & Klaentschi. This is how they describe it. "A retreat to work and contemplate the garden geometry. Right next to the Studio stands an ancient STONE which is a marker on the LINE: Stonehenge, Old Sarum, The Cathedral, Odstock and on to Clearbury Ring. The STUDIO like the STONE is a monolith speaking the language of form just like the globes of the topiary YEWS each all one material. The Studio is a rusting steel monolith with the side facing the causeway - the LINE is being constantly polished by the passing traffic, real or imagined. The drama of opening the super large door is equalled by the surprise of the birch plywood interior. There is sunlight streaming down from the gods. A sensual black walnut undulating desk resides flush with the long low window and just when you are frustrated at not being able to see out you comply with the perfectly designed seat and there the splendid garden panoramic view is presented to you. The workmanship and in particular the carefully designed objects within the room - stainless steel box providing a shower toilet, the tea sink and storage units are exquisitely made."