loft conversion

Do you mean for a dormer window and a staircase just? If this is to be a bedroom it will require new joists (almost certainly), 30min fire door, 30 min fire protection in floor [and walls] and a "means of escape window" at the front. The dormer window and staircase would be done with the joists normally. It would delend on the area of the country I guess but for a small single room loft conversion all in you would be looking at 20-25k I would think. Getting structure only, which would give you an unfloored unplasterboarded unwired shell with dormer, front escape window, fire door and staircase would be about 2/3 of this cost, I would say.

You would then have to provide insulation/fire protection, flooring, plasterboarding, skimming, wiring, fire alarms, door closers on internal doors to staircase (and replacement of any glazed ones). Which is what I did (successfully!)

Tis what you will get ;-)

Doing it yourself or having these bits done?

The materials for a dormer will be say £2k - depends on if this is a pretty road facing one with a tiled bonnet roof, or a less attractive flat roof jobbie. Just doing the dormer ought to take a builder no longer than a few days, especially if they are not having to actually finish the interior or make good. So say £4K as a ballpark, perhaps £5k with scaffolding.

Stairs, again much depends on type. You can make most flights (even complex jobbies with quarter winders) for a couple of hundred in materials. If you have a set made to order then expect to pay £600 - £1000. Ballustrading will cost extra. Say two to three days to fit, so you can probably extrapolate a typical cost from there.

Here is a set I did earlier, total cost probably about £400ish but that included redoing the ballustrade on the other flight to match the new ones