Thanks for the namecheck Jem :o)))))
Yes I've done this sort of thing before. "Mezzanine" floors are very popular right now. I guess you intend to have a "vaulted" ceiling ........... i.e. open right up to the roof structure?
Creating the floorspace would be a simple, not very costly, job.
I would consider spending the bulk of the money on, for instance, a simple glass wall to create a more "private" space upstairs. A self-contained room would add a lot more value.
Also ......... in the pic ......... I would go for a "proper" staircase ........... timber, steel, glass .... anything you like .......... and, unless you intend to make the stairs part of a "feature" on the ground floor, I would look at putting it against one wall ...... a few steps coming down from the Mezzanine ....... then possibly a quarter-landing and turning into the middle of the room with the remaining treads.
Price would depend on where you live .......... but I reckon what I described could be done within the 20K Jem mentioned. If it were done within a larger job of roof re-fit etc, then possibly well within that figure.
Don't quote me on that ............. you might have trés expensifs tastes :o)))