If in doubt always check with your local authority
An extension or addition to your house is considered to be permitted development, not requiring an application for planning permission, subject to the following limits and conditions:•No more than half the area of land around the "original house"* would be covered by additions or other buildings.•No extension forward of the principal elevation or side elevation fronting a highway.•No extension to be higher than the highest part of the roof.•Single-storey rear extension must not extend beyond the rear wall of the original house* by more than three metres if an attached house or by four metres if a detached house.•In addition, outside Article 2(3) designated land* and Sites of Special Scientific Interest the limit is increased to 6m if an attached house and 8m if a detached house until 30 May 2019.•These increased limits (between 3m and 6m and between 4m and 8m respectively) are subject to the prior notification of the proposal to the Local Planning Authority and the implementation of a neighbour consultation scheme. If objections are received, the proposal might not be allowed.•Maximum height of a single-storey rear extension of four metres.•Extensions of more than one storey must not extend beyond the rear wall of the original house* by more than three metres.•Maximum eaves height of an extension within two metres of the boundary of three metres.•Maximum eaves and ridge height of extension no higher than existing house.•Side extensions to be single storey with maximum height of four metres and width no more than half that of the original house.•Two-storey extensions no closer than seven metres to rear boundary.•Roof pitch of extensions higher than one storey to match existing house.•Materials to be similar in appearance to the existing house.•No verandas, balconies or raised platforms.•Upper-floor, side-facing windows to be obscure-glazed; any opening to be 1.7m above the floor.•On designated land* no permitted development for rear extensions of more than one storey.•On designated land no cladding of the exterior.•On designated land no side extensions.
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With all building work, the owner of the property (or land) in question is ultimately responsible for complying with the relevant planning rules and building regulations (regardless of the need to apply for planning permission and/or building regulations approval or not).Therefore, failure to comply with the relevant rules will result in the owner being liable for any remedial action (which could go as far as demolition and/or restoration). The general advice is to always discuss your proposals with the relevant Local Planning Authority and Building Control Service before starting work.