Hi - we've done a loft conversion in our current house and DH has done a couple in top-flr flats 10 years back.
Our loft conversion took place about a year ago and we added a master bedroom and en-suite - which we love! We got a specialist loft company to do the conversion for us and would recommend this route. As they specialise in lofts they have a tried-and-tested method and got the job done very quickly and to budget.
It took 7 weeks for the main build and then the tiling / decorating and carpeting was done afterwards. The loft company only do the main build and then we had to hire a tiler and decorator for the finishing jobs. They recommended people to us. The loft company also have in-house architects who have standard plans for lofts which means that the builders are used to building a particular type of loft.
Cost was c. £33k + VAT (but then need to factor cost for bathroom (basin, shower, etc) and also for carpet, tiling and decorator costs, etc. Basically the quote was just for the main building work. I think we got a good deal as they weren't busy and it was just after the credit crunch etc. We live in a terraced house in London.
Previously my DH had used general builders which didn't work as well - they didn't do as many lofts so took a lot longer. Doing it this way we had to hire an architect ourselves who then has to talk through plans with builders, etc.
Theres lots to think about - but most importantly you need the builder to check the head height of your loft when they come round to estimate, etc. If you do not have enough head height then they need to lower ceilings in your first floor which adds ££s and LOTS of dust! Also think about time of year - we did it at end of summer which was perfect as builders could get on with it in good weather and meant house wasn't freezing.
We didn't have any problems with planning permission as there are quite a few loft conversions already on our street - so precedence has been set.
PS Its the cheapest and easiest way to add value to your house.