The rights do not apply to:
buildings converted to residential under permitted development rights
conservation areas, areas of outstanding natural beauty, the Broads, national parks, world heritage sites and SSSIs
buildings constructed before 1 July 1948 or after 28 October 2018
where the existing dwelling house has been enlarged already by the addition of one or storeys whether under permitted development or otherwise
There are other restrictions:
following the works, the height of the highest part of the roof
cannot exceed 18m or the highest part of the roof as existing by more than 3.5m for one storey dwellings or 7m otherwise
cannot exceed by more than 3.5m the highest part of the roof of:
for semi-detached houses, the building with which it shares a party wall
for terraces, every other building in the same row
the internal floor to ceiling height of additional storeys cannot exceed the lower of 3m and the existing height of any storeys in the principal part of the existing house
the new storeys must only be constructed on the principal part of the house ie the main part, excluding extensions of lower height whether original or subsequent
no visible support structures on or attached to the exterior on completion are allowed
no windows are allowed in walls or roof slopes on side elevations and the roof pitch of the principal part must be the same
the materials used in exterior works must be of similar appearance to the existing
Consideration also needs to be given to the standard provisions of the General Permitted Development Order, including where existing permissions disapply permitted development rights.