It is in my loft.
I had already had a floor put in the loft. I built the 'shed' by erecting stud walls attached to the inner purlins. I found an old latch door on eBay and carpeted the inside all over (including walls and ceiling) using carpet tiles and off-cuts. Far end is plasterboard over brick, followed by closed-cell foam and then acoustic tiles to try to keep sound leaking to my good neighbours! At each side of the shed is a large storage area. The access hatch gives entry to an entrance- hall area. I could have done it all cheaper if I had planned it properly in advance, rather than buying the materials as I went along, but it kept me busy, and that's a good thing.