You will not require permission, provided that the proposed extension complies with Rules A - F below. Rule A. The size of the extension must not be more than - i. Terrace house • 50 cubic metres or 10% of the original house volume, whichever is the greater. ii. Any house in a Conservation Area • 50 cubic metres or 10% of the original house volume, whichever is the greater. iii. Any other house (detached or semi-detached) • 70 cubic metres or 15% of the original house volume, whichever is the greater. Note 1 - In all cases size is calculated from external measurement, and there is an upper limit of 115 cubic metres.   Note 2 - Any extension built since 1 October 1973 will not be treated as part of the original house volume. It will therefore be necessary to subtract the volume of any existing extension from the allowances specified above. The amount left over (if any) is the amount which you can extend without planning permission providing the proposed addition also complies with Rules B to F (see Illustration 1).   Illustration 1 Building an Extension (Note 2 & Note 3) Three identical extensions but only the extension at house ‘A’ would not require planning permission. At house ‘C’, the existing garage was added after 1 October 1973 and is therefore treated as an extension. The volume of the garage (44m3) plus the proposed extension (30m3) would exceed the stated allowance (70m3) and so planning permission would be required. At house ‘B’, planning permission would be required because the extension would be within 5 metres of an existing building belonging to the house, causing that building to be treated as if it were part of the extension. The combined volume again exceeds the stated allowance. Finally, at house ‘A’, the existing garage would not be treated as an extension as it would remain beyond 5 metres from the house and the proposed extension.

Rule B. The design and materials to be used to build the proposed extension must be in keeping with the main house.

Rule C. No part of the extension should be nearer to any road than the part of the original house nearest to the road.

Rule D. No part of the extension should be higher than the highest part of the existing roof of the house.

Rule E. The extension should not be more than 4 metres high if it comes within 3 metres of the boundary of the property (see Illustration 2). Height is to be measured from ground level, measuring the level of the surface of the ground immediately adjacent to the building or, if it is not uniform, the level at its highest adjacent part.

Rule F. The ground area covered by the extension and any other buildings within the boundary of the property (excluding the original house) must not be more than half the total area (not counting the ground area of the original house).

Illustration 2 Building an Extension (Rule E)

Calculating the volume of extensions and roof space extensions can be difficult. To make this process easier, Planning Service have provided a Volume Calculator Courtesy of the The Planning Portal www.planningportal.gov.uk

The Volume Calculator requires Macromedia Flash 5.
Click here to go to Macromedia's web site.