Hi,
Just found this site on a search...
I've converted my house into two flats.. I'm at the final stages now, awating my final walk round for building regs and my sound proof test..
Not sure if I'd suggest it to the faint hearted.. I've done most of the work myself and it's took about two years .. bit by bit..
Some of the things you need to think about to do this work are:-
1. You'll need planning permission, if you try and do it on the sly, (as some have suggested) you'll not be able to let it out, will have insurance problems, and could seriously devalue the house, not to mention be at risk of prosicution at some point.
2. the new Building regs require allot of sound proofing, and require an independant test to check it complies with the regulations - I've got suspended ceilings with acoutic brackets, two layers of plaster boards, sealed witgh acoustic sealant, then upstairs is sealed and another layer of board is put over the existing boards.
3. You'll need approx £700 for planning with an architect, then £600 for building regs dependant on size
4. once it's all done you'll need to sort out a lease hold - I'm looking into this now.. as it causes problems with your existing mortgage once it's changed over..
It's been a big learning curve but can increase the value of the property for a quick turn around if you do it right...