Planning permission is not normally required. However, permission is required where you extend or alter the roof space and it exceeds specified limits and conditions.
A loft conversion for your home is considered to be permitted development, not requiring an application for planning permission, subject to the following limits and 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 and conservation areas.
Building Regulations drawings and full buildings regulations approval will be required on a loft conversion.