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 house 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 additional roof space for terraced houses
A volume allowance of 50 cubic metres additional roof space 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 original eaves
The roof enlargement cannot overhang the outer face of the wall of the original house
Planning permission is not usually required, providing the work is internal and does not involve enlarging the building.
Sometimes permitted development rights have been removed from some properties with regard to garage conversions and therefore you should contact your local planning authority, Ashford Borough Council before proceeding, particularly if you live on a new housing development or in a conservation area.