As has been mentioned several times, if it appears there is an ongoing risk to life for example the floor is only 125mm thick and has obviously had no reinforcement whatsoever or there is no separation between the loft and the first floor and he feels its in regular use he may take enforcement action. At the very least he will mark you down on his system as having a non compliant (illegal) loft conversion. Unless it looks bad he'll probably leave it at that having written to you.
However when you come to sell the buyers solicitor will ask you for a Building Regs Certificate, when you cannot produce one the buyer will probably walk away unless you are prepared to get retrospective approval. This will also pop up in the searches. If anyone including yourself wants to do any more compliant work on the property and get Approval they will hold the loft to ransom. Whilst this is not strictly legal for them to do this I have witnessed this practice going on.
However when you come to sell the buyers solicitor will ask you for a Building Regs Certificate, when you cannot produce one the buyer will probably walk away unless you are prepared to get retrospective approval. This will also pop up in the searches. If anyone including yourself wants to do any more compliant work on the property and get Approval they will hold the loft to ransom. Whilst this is not strictly legal for them to do this I have witnessed this practice going on.