i created my own posh shed in our back garden, with damp proof membrane, ventilated floor, insulated walls and clad in cedar.
Being a 'hands on architect' i built it myself, but bought some french doors on line which simply dropped in to the frame.
Of course depending on how large you want to go - you should check any planning issues (we had to get retrospective planning for a client that built a garden summer house a little too high last year). You also need to be mindful of Building Regulations - even party walls could kick in if you are building close to boundaries - depending on how 'cool' a design you want etc.
We could help you with the design, offer suitable cool ideas and suppliers of doors/bifolds/green roofs whatever you aspire for. it would be something we would love to help with.
www.atelierdb.com - we are a small office in SW18 and we provide private homeowners with some cool affordable design solutions.
I do know a local contractor who has undertaken these types of garden buildings, but usually only as part of a larger contract.