I have one, there are a number of things I'd do differently this time. Mostly around the edges of the build. Cost I guess will vary by region. Now its finished I'm more then happy we did it.
Things I like about it
Build generally went well. Took about 6 weeks and people were onsite when they said they'd be every day. The worked well, and it was only really disruptive when they broke through the existing ceiling and put the new stairs in. It took around 4 days for this and we just went away so they could leave the existing doors sealed. We'd also planned to replace the carpets in the hall, and I doubt the old ones could of been cleaned afterwards
The architect was also fantastic.
Things I think could be better
Planning permission. I know the laws have changed since we had it done, and because of the location of our house we needed planning permission. Unfortunatly the guy from the loft company dealing with the local planners was best described as a cock, who went for a design that only ever stood a marginal chance of passing planning. He also annoyed the planning girl so much she basically wrote a report for the council damning it so it was never going to be accepted. After the first debacle the architect and I took a far more sympathetic design and discussed with planners at length prior to submitting it. It was then accepted without condition. This cost us 6 months which as we'd planned to have this finished before my daughter was born wasn't ideal.
The plumbers. Were not very good and a number of corrections to their work had to be made afterwards. They also flooded the house and have caused ongoing issues, including pumping showers direct from mains water. It was a good job we caught this when we did.
The plumbers were sub contracted and it took alot of effort to get the loft company to take responsibility for their actions.
HTH
H