We finished ours a couple of months ago and it's so, so worth the hassle (and that's from someone who works from home and so couldn't escape the banging all day).
Can you give us a few more details on what you're having done?
I don't know if I'm the least house proud person that ever lived... but I didn't find the dust to be such an issue.
Things we 'over' spent on: £250 for deciding that we'd prefer render and paint on the dormer not 'shiplap' weatherboarding. £200 for fitted shelves. £20 for a better spec. of radiator. £500 mist, undercoat, woodwork and topcoat painting. £20 for safety catches on the windows.
Our builder gave us a fixed price, but I'd put some contingency money aside anyway, just in case. This ended up paying for the overspends above as well as carpet for the new loft room, hall stairs and landings and the other 4 bedrooms of the house... so all in all, I was pleased with the final spend.
I'd make sure you know exactly what finish the builder is going for. You may have beautiful wooden doors in mind - the builder is probably thinking £40 pre-painted, molded chip-board.
I found working with photos really helped. When we were planning the built in cupboards I printed off a couple of pictures from websites and the builder was able to make ones exactly the same. It saved days of 'kind of like this, but more like this' discussions.
I'd also say hold out for what you want. We had an odd 'v' shaped chimney breast in the attic that, for ease, the builder just wanted to plasterboard over. I held out to keep the shape in the room. We were able to build in shelves around it and it looks lovely - so much better than just a boring, modern-box shaped room.
Finally (because I'm on a roll now
) I'd say beware of project creep.
Because we had carpenters and sparks in the house we found a huge list of odd jobs in the rest of the house. We ended up replacing banisters on the stairs, fixing some woodwork, having a new doors made, having another door shortened, installing two new light fittings in the kitchen and having another fitting moved in the hall. Not part of the original project but all adding up. Along with extra decorating, our 'overspend' came in at £2700.