Also check what footings you need - we had to have a raft cast in two sections (on a hill) which added greatly to the cost.
£100 to 120 per sq ft footprint is about right given the spec, but a lot will depend on cost of materials at the time as they fluctuate, and how available/busy your builder is.
Yes was fairly simple - about a days work. We got it from Permaroof, but there's a lot of companies supplying it. They gave us a kit with the adhesives, flashing and all the edge trims. We built the roof with a slight run off and then basically deck-adhesived the rubber down with contact adhesive around the edges. You use some special bits of rubber on the corners which you vulcanise and stretch around. All in all its very satisfying and only cost about £150 for materials (6x2 sq meters). http://www.permaroof.co.uk/ Have a look at this link, make sure you order plenty for overhangs etc and research all the videos telling you how to do it all. We are very happy with it, it's not perfect (a few air bubbles in places) but it does what it says on the tin.
As it's so small, it shouldn't cost much more to make something more interesting than a plain square brick extension with a flat roof. I wouldn't be able to resist personally.
I think my ex father-in-law probably made a profit building his extension, that's the kind of guy he is.
If you were canny it could be cheaper. Timber clad wood frame blah. Less weight, less foundation? Etc.
