Under permitted development you are given certain allowances for extending your property. If you live in a conservation area or your house is a listed building your permitted development rights may be restricted under article 4 of the GPDO, if you are unsure check with your local council who will be able to advise you. Loft conversions are considered permitted development subject to the following conditions:
• A volume allowance of 40 cubic metres for terraced houses.
• A volume allowance of 50 cubic metres for detached and semi-detached houses.
• No extension beyond the plane of the existing roof slope of the principal elevation that fronts the highway.
• No extension to be higher than the highest part of the roof.
• Materials to be similar in appearance to the existing house.
• No verandas, balconies or raised platforms.
• Side-facing windows to be obscure-glazed; any opening to be 1.7m above the floor.
• Roof extensions not to be permitted development in designated areas*.
• Roof extensions, apart from hip to gable ones, to be set back, as far as practicable, at least 20cm from the eaves.
*Designated areas include national parks and the Broads, Areas of Outstanding Natural
Beauty, conservation areas and World Heritage Sites.
A new interactive section on the planning portal website offers an excellent visual guide to the new rules.
Note that you should always contact your local planning authority to discuss your proposal before any work begins to confirm that the development is permitted.