Single storey extensions and two-storey extensions sometimes can be considered to be permitted development provided that it meets certain conditions.
Generally speaking, you may not need to apply for planning permission if the proposal meets the following criteria
• the extension (including previous extensions) would not exceed 50% of the total area of land around the original house.
• the extension would not exceed the height of the highest part of the roof and the eaves of the extension would not be higher than the height of the eaves of the house.
• the extension would not be set forward of the principal elevation or side elevation of the house and fronting a highway.
• the extension would not project beyond the rear wall of the original house by more than 4 metres if it is a detached house, or by 3 metres if it is other type of house.
• the height of a side extension would not be higher than four metres above ground level.
• the extension would be less than 3 metres deep and it would not be within 7 metres from the rear site boundary and the extension is more than one storey.
• the eaves height of an extension would not be more than 3 metres and it is within 2 metres from the boundary.
• the side extension would not be more than 4 metres in height, and its width would be less than half the width of the original house and it would be more than one storey.
• external materials to be used would be similar to those of the existing house, however will not apply when the extension is a conservatory.
• any upper-floor window in a wall or roof slope in a side elevation would be obscure-glazed and non-opening unless the parts which can be opened are more than 1.7 metres above the floor of the room in which it is installed.
• the roof pitch of the extension would match the roof pitch of the original house, if the extension is more than one storey.
If your house is within a designated land, you will need to apply for planning permission for side extension and two storey rear extension.
* Designated land includes national parks and the Broads, Areas of Outstanding Natural Beauty, conservation areas and World Heritage Sites.