If the conversion can be done under your home’s PD rights, it’s a good idea to apply to your local council for a lawful development certificate, which is for building work that doesn’t need planning permission.
When you come to sell your home, this certificate proves to the buyer and their solicitor that the work is lawful. ON ‘DESIGNATED land’, which includes conservation areas and ‘Areas of Outstanding Natural Beauty’, loft conversions are not permitted development, so you have to apply for special planning permission. The same applies if you live in a flat or maisonette, as they don’t have PD rights either.
With listed buildings, you need listed building consent from your local council to do building work, including a loft conversion.