Planning permission is not normally required. However, permission is required where you extend the roof space, and it exceeds specified limits and conditions.

Read more about loft conversions.

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The permitted development allowances described here apply to houses and not to flats and maisonettes.
Read about loft conversions for flats.

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Buildings regulations apply if you want to carry out a loft conversion to your home.

The appropriate requirements of the regulations will be applied to ensure, for example: the structural strength of the new floor is sufficient; the stability of the structure (including the existing roof) is not endangered; safe escape from fire; safely designed stairs to the new floor; and reasonable sound insulation between the conversion and the rooms below.

You will also need to consider whether your loft conversion project is subject to The Party Wall etc. Act 1996 under which you must give adjoining owners notice.

Protected Species

Although planning permission is not required, a wildlife licence may be required if the work will affect protected species like bats.