Hi, we had a dormer fitted, full staircase, bathroom - its a big bedroom and guests love it.
Including all decorating, the stairs etc it cost about £34k, it was £29k to do the work and then £5k for bathroom, decorating etc
My top tips:
-make sure you get a written costing of everything thats included. E.g in our case, the costing didn't include the stair case which cost an extra £900.
-fire doors - you may need these fitted throughout the house - we had lovely original victorian ones that had to be replaced (sad face) and the new ones were around £50 each x 8 = more money!
-after the conversion, we had to redecorate the hall and stairs and buy a new carpet for stairs due to damage caused during the conversion even though our builders were careful - so cost that in and dont do anything on moving in to hall and stairs until you have finished conversion
-keep some space (perhaps in eaves on one side if you have dormer on another) for storage
-I definiately recommend dormer for hte extra space even if it costs more - you will prob regret not paying the extra for it
-be prepared for mess, dust and more mess and and dust - and for having no roof for a while
-when getting quotes, consider time it will take as well as cost. we went for a cheaper quote but it was a one man band doing it and it took a lot longer than anticipated
-ensure in costings that they take everything away (e.g skip)
-try to be on holiday when they 'break through' !
-dont do it when you have 2 x 8 month old babies
good luck!