Can my attic conversion be counted as a room when it comes time to sell?

Property Advice: Definition of an attic conversion as storage or an habitable space

Hello Brigid

We bought a 1960s-era semi-detached house 15 years ago that had a really good attic conversion done to it back in 1979. While planning permission was granted for the conversion, it doesn’t meet the technical requirements to be described as a bedroom/ ‘habitable space’. We knew that when buying, of course. We loved the scope, and it has been a top-level room for us, main bedroom, en suite, and office.
If we decide to sell at some stage, to trade down, we’d like prospective buyers to appreciate the extra space (there’s 450 sq ft up here) and the rooms, but ‘officially’ we’re told it has to be described as ‘storage’. How big a deal is it to get it up to meet building regs in terms of fire proofing, fire doors, smoke/fire detectors, and we’re told, ‘double slabbed ceilings’?
It would add to the value, surely, on a sale. But won’t others see it for what it is, like we did?
Fiona and Tim