If your property has full permitted development rights (PD) attached to it, then you will have the provision to extend over two storeys to the rear of your house. It is important to check with your local planning department whether you have PD in place.

Permitted development rights are very powerful and can enable you to make changes that planners would never allow, but it’s vital to have your rights checked by an expert.

How big can your two storey extension be?

Extensions, including those already built, and other buildings must not exceed 50 per cent of the total area of land around the original house. The term “original house” means the house as it was when it was first built, or as it stood on 1 July 1948.

  • Extensions greater than one storey must not extend beyond the rear wall of the original house by more than three metres
  • Two storey extensions can’t be within seven metres of any boundary opposite the rear wall of the house
  • The eaves height of a rear extension cannot be greater than three metres if it is within two metres of an adjoining boundary, so generally a two storey extension cannot be any closer to the boundary than this
  • No two storey extensions can be made to the front of the house, or to any wall parallel to the highway

When extending at the rear, beware of technical traps. For detached houses, you can extend a single storey out four metres from the rear wall, whereas two-storey extensions are limited to three metres in depth.

Don’t be fooled into thinking that you can extend the ground floor by four metres, but with a three-metre projection on the first floor. If the two elements are connected then the whole structure must not exceed three metres in depth.

Two storey extension roof design

In most cases, homeowners planning a two-storey extension under permitted development will want to soften the visual impact of adding such a large volume to their house by incorporating a pitched roof into the design. One aspect of permitted develoment that often catches people out is roof design.

  • If the roof of the extension connects to your existing roof you will need to deduct the volume of the extension roof from any volume allowance you have in place for a loft conversion.
  • For attached houses there is a loft-dormer allowance of 40 cubic metres, and 50 cubic metres for detached properties.
  • Under PD, a two-storey extension could have a roof element of 10 to 35 cubic metres.

First floor windows

Any upper-floor window in a wall or roof slope of a side elevation must be non-opening and obscure-glazed, unless the opening parts are more than 1.7 metres above the floor of the room in which it is installed.

What if my house is terraced?

Terraced properties benefit from a PD allowance for two-storey extensions, but this typically fails to deliver any spatial benefit when the two-metre boundary rule is taken into account. There are exceptions to this rule, but they are very rare. The key PD benefits relate to detached houses and large semi-detached properties.

For more information visit the Government’s planning website: planningportal.gov.uk.

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