Volume limits

NOTE: This guidance applies only until 1 October 2008 when new permitted development rules and conditions come into force.

You will need to apply for planning permission before building an extension if:

• for a terraced house (including an end of terrace house) or any house in a Conservation Area, National Park, an Area of Outstanding Natural Beauty or the Broads - the volume of the "original house" would be increased by more than 10% or 50 cubic metres (whichever is the greater);
• for any other kind of house outside those areas, the volume of the "original house" would be increased by more than 15% or 70 cubic metres (whichever is the greater); and
• in any case, the volume of the "original house" would be increased by more than 115 cubic metres.

Volume is calculated from the external dimensions of the entire structure of the extension.

In the following circumstances, the volume of other buildings which belong to your house (such as a garage or shed) will count against the volume allowances. In some cases, this can include buildings which were built at the same time as the house or existed on 1 July 1948.

• If an extension to your house comes within 5 metres of another building belonging to your house, the volume of that building counts against the allowance for additions and extensions.
• If you live in a Conservation Area, a National Park, an Area of Outstanding Natural Beauty or the Broads, and the building you wish to add would be more than 10 cubic metres in volume, it will (regardless of where situated in relation to the house) be treated as an extension of the house, and reduce the allowance for futher extensions.
• Elsewhere, if you add to your property any building more than 10 cubic metres in volume and within 5 metres of your house, it will be treated as an extension of the house, and reduce the allowance for further extensions.

If any of these cases apply, the volume of the building concerned will be deducted from your volume limit for extensions and additions to your house. For example, if your volume limit is 50 cubic metres and a building of 15 cubic metres in volume is treated as an extension to the house, then your volume limit for extensions would be reduced to 35 cubic metres.

Calculating the volume of a building or extension is not easy. It is important to get it right if you are deciding whether planning permission will be needed. To help you we have provided a handy calculator.

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