Don't know anything on the legal side, but I bought a place back in 2010 with a non-approved loft conversion (it was a bargain

), and spent a good while stripping it out and redoing it myself to get building control approval, so I know a fair bit on the construction side.
The reality is that it's unlikely to be something small that can be rectified to gain approval, it usually requires significant modification or stripping completely. There's a handful of major requirements people usually fail to meet so go with the easy non-approved route...
- Floor structure - new joists/steels required , which can take away an unacceptable amount of headroom.
- Stairs - there are requirements on angle, head height etc. that can be difficult to meet.
- Fire protection - As well as the loft itself, there must be a fire protected stairwell to a ground floor exit, again often not possible (e.g. stairwells opening into living rooms)
- Fire egress - There has to be a window which can be accessed with a ladder, roof structure often prevents this
It's likely one or more of these it fails on, some councils are flexible however, e.g. I've heard of the fire egress requirements being dropped in exchange for increasing fire resistance from 30 minutes to 60 minutes.
FYI it cost me about £13k (and a lot of time) to sort my house, I would expect ~£20k if I paid someone to do it, that's for a 6x6m conversion.