I did a sort of loft conversion -the loft was adjacent to my flat rather than above it as I am in the attic
I got an architect to draw up plans – unfortunately he made mistakes in the spec which meant altho I built it to the spec it did not meet building regs. Mind you the building regs chap from the council approved the plans but then found the fault after I built it to the spec when it was inspected so he made mistakes as well. The structural engineer had to be told which walls were load bearing and which were not – he wanted me to put a massive steel in to replace a non loadbearing wall and I had to show him that nothing was resting ontop
I used a builder to do the structural work and did all the interior myself which meant I could make sure that fittings went where I wanted them to go. Unfortunately the builders put one of the windows in the wrong place – it was in the right place on the plans but they decided it would look better where it was.
I massively over ordered on the plasterboard as well. about 30% over estimate
all in all I got £20 000 worth of work for £7000 cash and 3 months of my sparetime.
By acting as main contractor yourself you save money – and I did a lot of labour which saves money as well. However its a lot of work. I used a the builder for the bits where I felt an insured professional was needed and then did the rest myself employing the likes of plasterers / gas fitters on a day rate when needed.
If I did it again I would check he architects work more thoroughly – I trusted him to get the spec right. I would research the correct spec myself. Other wise I would do much as I did.
The other thing I did was to make sure I got all the fixtures and fittings exactly where I wanted – including loudspeakers and lights – DIY makes that easier.
chose your professionals well. get personal recommendations if you can