The short answer

If the loft becomes habitable space, the work needs Building Regulations approval. There is no homeowner exception, no 'small enough' exception, and no 'just for me' exception.

How unauthorised conversions surface

Sale, remortgage, insurance claim, neighbour complaint. Any of these can trigger questions you cannot answer without paperwork.

The fix

A Regularisation application. The Building Control body assesses the existing work against the Regulations — usually requiring opening up — and issues a Regularisation Certificate if it meets standards. It is more expensive and more disruptive than doing it properly first time.