I keep extending mine, and use 90 x 40 ish studding for the basic frame as its cheap compared to normal sawn timber and you can get treated from b&q, then white upvc cladding on the outside which is also cheap at just over £10 for a bit 300mm high by five meters long, then insulate and clad on the inside with pine. I also double glaze, and while my last windows were proper upvc ones from b&q I found it cheaper to go to a local glazing firm and for a double glazed glass unit 60 x 45cm it was only £25 then build a wooden frame yourself. The windows and insulation do make a huge difference as it keeps it cool in the summer and warm in the winter, and I have a heater which is on a timer so it heats it up for the day, but even off on the coldest nights we've had it doesn't drop below 11 degrees inside. If you can build yourself is so much cheaper for what you get. Just remember a nice solid concrete base a couple of rows of engineering bricks then place damp course plastic on top to sit the wood on.

:) Don