I work from a "shed" at the bottom of the garden. 22ft x 12 ft.
We got it from our local garden shed place, but on their advice ordered it double skinned with insulation and double glazed. Since we did this we started noticing the log cabin office ads in the weekend colour supplements, but try your local garden shed place.
We had to clear the area and lay the foundations to their spec (well i really got someone in to do it), they came with the sections and built it in one day. Weobviously had toget the shed people, electricians and plumbers to co-ordinate their efforts.
Then we carpeted, curtained and furnished it.
It is serviced with electricity, washing up basin, loo - pumped back to the mains drainage, electricity and phones. I expect I could put a power shower in by giving up a little bit of space.
In the summer we are probably cooler than most of you out there because it is sheltered by trees. But even if it was not we could have fans and or air con.
As far as the winter is concerned we have a small electric heater at each end of the office, and we are only cold when i have left it on too low overnight and we have a cold one. Once we have sorted that, then the place can be as roasting hot as you want, and I have no doubt you could get a more sophisticated, thermostatically copntrolled system - we manage .
Thinking of all the therapy rooms I have been theraped in, then it is probably two to three times the average size.