We did a loft extension about 7 years ago - the best thing we've ever done to the house. We went from 3 bed to 4 and added a decent size separate bathroom. It cost around £50k including all bathroom fittings, tiling etc. and decorating. I've mostly heard only good things of the various loft companies in the area. Building control is very strict which means that the companies have to do a good job, by and large. The company will do all the liaison with them and it will need to be signed off as well.

Main tip from me is that time invested in the design is definately time well spent. Oh, and do get friendly with the neighbours. It is a huge and messy job and will really affect them as well, so one of your 200 bottles of wine might not go amiss as a present!