-
Whoever is telling you this isn’t an issue is misinformed.
You have a duty to truthfully reply to anything you are asked by the buyer. If you don’t and are later found out to have been dishonest (this post would be sufficient evidence that you knew about the loft), you can be made liable for any costs associated with putting right or the sale can even be reversed.
Most buyers would be put off buying a property which is possibly built to sub-building regulations standard. A buyer’s solicitor would have to be a bit of a baffoon to miss an unregulated extension.
Your best bet is to regularise the works via the council’s building control (can’t go private for regularisation).
If you are set on selling, I would do this ASAP. The council will likely ask you to open up walls, floors and ceilings for inspections. Put some money aside for this – even where you don’t need to undertake remedial works you will need to snag all the openings.