I did it myself, with help from a qualified builder. Point was that we didn't make it into a formally recognised room - as Saracen stated, there are so many regs esp around the permanent staircase/ firedoor etc, for which there wasn't space. Didn't move the struts, since was do-able but going to cost a whole heap. I had to make it usable within 2 weeks while my wife was away,
lol.
New ceiling insulation, loft window installed, standing desk/s, plasterboarding etc
When we come to sell the place, will simply be a glorified loft, so not added REAL value, but turned it into my office. Made space. Cost was minimal.