Mark,
Ah, yes been here
I received permission from higher powers to build a home for the cars - by the time I had finished I had created a monster
I looked at all manner of modular builds from eBay but in the end found a local master carpenter - we cleared the site which is 38'x20' and then excavated down to create the pad - great fun learning how to drive the mini digger

this was filled with hardcore sourced from local projects and then compacted (the whacker plate really kills your arms

).
Running in the concrete was interesting (we had the worst frost that night! Thankfully the inhibitor really saved our bacon. When dry a course of three bricks was run around the perimeter and then a dpc run around the top of the brick. Wood sections were created which enabled sheets of celotex or kingspan to be inserted into the gaps. The outer was then clad with a membrane for water retardence and then clad with a ship lap wood planking. The celotex has one inch of air between it and the outer and then provides a further three inches of insulation - total of five so in excess of building regs.
A local truss manufacturer made the sectional spans which were then covered with membrane and finally - and this was over the top but looks so beautiful - welsh blue slates reclaimed and purchased off eBay or local salvage yard. Main reason other than looks was in case of a firework landing on a clad roof
Foil backed plaster board on the inside and then a local plasterer did an amazing finish in one day (all on stilts - quite incredible to watch

).
Huge soak away constructed with aqua cell units to comply with regs, power was already down there so installed a separate consumer unit which was 40 quid off eBay complete with trips

- the doors are constructed in the same manner as the walls so require girt big hinges and a piece of steel over each opening but this was really a luxury. I installed the new earth to comply with pme and finished painting it a week after it was completed at the beginning of last year.
I'll post some pictures but previous poster is quite right - I have NO heat source - just the heat from two dehumidifiers which were sized to be precise for the space and a wine fridge which is very low energy and this will keep the ambient temperature in there at about +8c even when had minus 10 down here a week or two ago. It's also beautifully cool in summer.
It was huge fun building it - I'll try and post some pictures tomorrow - the process really opened my eyes to timber framed construction and constantly amazes me how energy efficient it is - I would love one day to build a house in the same way from that which I've learned - and can't understand the snobbery that seems to exist in this country that it's "not brick"
Hth
P