Updated: Oct 23, 2019
Permitted Development vs Planning Permission
A loft conversion is considered to be Permitted Development, not requiring an application for Planning Permission, provided that certain limits and conditions are met.
To be a Permitted Development any additional roof space created must not exceed following volume allowances:
40m3 on terraced or semi-detached houses
50m3 on detached houses
Apart from that your proposed loft extension should use materials, which appear similar to the existing property. It also cannot be higher than the highest point on the existing roof.
Permitted Developments for loft conversions would not include balconies, verandas or raised platforms. It would also not be permitted if the proposal faces front of the road.
A side window on your loft extension should be obscured and not able to be opened, unless the opening mechanism is situated at least 1.7m above its respective room floor.
Roof extensions should be set back, as far as reasonably feasible by at least 25 cm from the eaves, unless it is a hip or gable roof. The roof extension should not hang over the wall of the house.