I've just built a single storey extension (6m x 5m) on the back of my house, I did most of the work myself but got a couple of brickies in,a joiner and electrician. Just need to do skirting and final fix for electrician.
Rough prices are:
Architect £220
Brickies £2500
Roof materials £1000
2 x Electric Velux windows £1000
Building regs £250
Folding doors £3000
Joiner plus supply windows and joists etc for roof £2000
Materials £4000
Plastering £800
Electrician and materials £500
Floor screed £700
Underfloor heating £600 (cost £1600 but is doing 3 rooms)
Marble tiles £1000
It's coming in at about £20,000.