Class B of The Town and Country Planning (General Permitted Development) (England) Order 2015 permits the addition or alteration to a roof of a house, subject to limitations and conditions. 

Some of the limitations are:

  • The dormer must not exceed the highest part of the roof of the house.
  • The dormer cannot be erected on the front of the house.
  • The cubic content of the roof space must not exceed 40 cubic metres for a terrace house, and 50 cubic metres for any other house.

Some of the conditions are:

  • The materials used in the exterior work must be similar to the existing house.
  • The dormer must be set back from the eaves by 0.2 metres.
  • Any window on a side elevation must be obscured and non-opening below 1.7 metres of the floor of the room.

Please refer to this legislation.