Double glazed windows and door, 2" insulation.
Working from home, I used to use a spare bedroom as an office, but when our second child came along I had to make other plans. The obvious solution was to buy a garden office, but none of the ones I looked at seemed to suit my needs, and were very overpriced. In the end my father persuaded me to reluctantly build my own. This is what I ended up with, at a cost of around ?6000. Its about 12 foot by 8ft with double glazed windows and door and uPVC soffits fascias and guttering. Exterior grade plywood was fixed to the outside of the timber structure, and the cladding was then nailed to this, with a waterproof membrane in between. 2" polystyrene was fixed into every available gap in the timber structure for insulation (roof, ceiling and walls), and MDF panels were used to line the walls on the inside. Electrics are provided via underground armoured cable from the house. I started the project in May 2005, and moved into the office in January 2006, but I spent most of my evenings and weekends inbetween building it. The only thing I didn't do was the concrete base (my arm was in plaster at the time). I've now been in the office for well over a year, and it has proved to be completely waterproof. In hindsight, the only change I would have made would have been to include underfloor heating - the air at the very bottom of the office can be quite cold, but the rest of the office warm (because I'm using a convector heater and the thermostat cuts out when the warm air reaches it from above). This was easily solved with a little skirting heater, though. I couldn't be happier with the office (which we nicknamed the "Beach Hut" because of the colour), but I'm not sure I'd undertake such a big project again.