Bought a house with a sunroom extension that would have been exempt. Don't recall our solicitor looking for any documentation or us being supplied with any.
In most cases when you come round to selling a house your solicitor or the solicitor acting for the purchaser will look for details of any extensions built to the house. They wont take your word that it was exempt so you can do one of two things.
Submit a sketch of the sunroom to the local planning dept. and ask them for a certificate confirming that it is exempted development (there is a fee for this) or you could get a local architect or engineer to give you a certificate confirming this.