If you fancy building your own garden office, consider using cob, a very old building technique which combines earth, straw, sand and water in a sturdy way to form lumps ('cobs') which you then press together to form walls. As long as it's not too rainy, you'll be fine and many people live in cob homes in the UK, especially in Cornwall and Devon. And of course it's superduper eco-friendly. Devon-based Cob experts Earthed - artists and builders who create sustainable structures specialising in cob, roundpole timber frame, lime and stonework - run educational workshops teaching you the basics of cobbuilding so you can create something like the above Wythenshawe cob building. For more smashing pictures click here.
Another option is Cob in Cornwall run from Helston by Adam Weismann and Katy Bryce who design and construct new cob and strawbale walls and buildings - an example of a studio build is pictured below.
As with the Earthed site, there's plenty of information on their web site to convince you that cob is the way to go. Here they are on heating:
"Earth buidings have outstanding thermal capabilities. The walls are generally built up to two feet thick, making it an ideal material for both cool and warm climates. These properties make cob an ideal material for creating garden courtyard spaces. The ability of the earth to absorb and then re-radiate the heat of the sun can create a micro-climate suitable for growing tropical fruit trees and for personal comfort."Katy and Adam have also written a book on building with cob which you can read about here and see a sample chapter. And below is a picture of them at work.