Why the room stops counting
A loft conversion needs building regulations approval, and the approval is what makes the result a habitable room. Without it the space is, in valuation terms, a loft — insulated, boarded, carpeted and furnished, but still a loft.
That has a price consequence you can measure. If the property was marketed and valued as having an extra bedroom, part of what you are paying for is a room that is not formally a room. It will be a loft again when you come to sell, and your buyer's solicitor will ask the same question.