This could include an extension to a dwellinghouse through changing the roof shape such as a hip-to-gable conversion, a dormer or forming a mansard roof; adding roof lights; or adding, altering or replacing anything else to the roof such as a solar panel, chimney, flue, or soil or vent pipe. This will not require planning permission provided, for an extension:
- The highest part of the extension is not higher than the highest point of the existing roof;
- It does not project forward of the plane of the existing roof slope on the principal elevation if that elevation faces a highway;
- The total volume of extensions added to the roof does not exceed the volume of the roof of the house as originally built by more than 40 cubic metres if the house is terraced or 50 cubic metres in any other case;
- It does not consist of a verandah, balcony or raised platform;
- Except for a hip-to-gable conversion, the extension must be set in at least 20 centimetres from the eaves, i.e. the bottom of the roof where it overhangs, unless this is not practicable;
- It is not in a National Park, an Area of Outstanding Natural Beauty, a Conservation Area, an area designated under section 41(3) of the Wildlife and Countryside Act 1981, the Norfolk Broads or a World Heritage Site;
- It is built using external materials similar in appearance to those on the existing house;
- Any window on the side of the house is obscure glazed and non-opening unless the parts of the window which can be opened are more than 1.7 metres above the floor of the room in which the window is installed.
For roof lights or any other alteration to the roof:
- The alteration does not protrude more 15 centimetres (20 centimetres for solar panels) beyond the original roof plane;
- It is not higher than the highest part of the original roof.
A chimney, flue or soil and vent pipe must not:
- be 1 metre or higher than the roof;
- must not be on a wall or roof slope which fronts a highway or is the main elevation of the house if it lies in a conservation area or other special types of area.
See also Property Alteration Overview for more potential restrictions: https://planningaid.zendesk.com/hc/en-us/articles/203210341-Property-Alterations-Overview