Permitted Development rights are a national grant of planning consent which allow certain building works and changes of use to be carried out without having to make a planning application. Permitted Development rights are subject to conditions and limitations to control impacts and to protect local amenity.
Whilst Permitted Development removes the requirement to apply for formal planning consent Andrews Eades would still recommend that you apply for a Certificate of Lawful Development from the Local Authority, as this would confirm that your proposed works are legal in planning terms.