just spotted this today ! Was there always a DIY forum ?
Anyways got an attic conversion done last year. Can't advise on cost as i'm not in the UK.
A few things that come to mind that may help . . .
(I'm not a builder so will stand corrected on any points below)
Stairs ! We lost no space in box room or boiler as builder built twisty stairs. Hard to describe but 2 turn stairs (Not spiral). So up a few steps turn right on a spiral 4 or 5 steps and few more steps straight in.
We kept it open plan rather than getting a door and walls separating loft from stairs and floor beneath.
Make sure your new loft floor is on steel supports. The steel will be lodged into your gable walls. If a semi-d they will be bolted using chemical screws into your neighbours joining wall. The new floor will sit happily on this. Some builders try to leave the steel bit out
Get plenty of velux windows in to create plenty of light.
Overall we were delighted with the new space !!
Good luck with it
