Source: www.the-mmc.co.uk (accessed 28 August 2009)
Planning Permission (as per September 2008):
Planning permission is not required provided that the proposed extension complies with Rules A - F below.
The size of the extension must not be more than the following:
Terrace house - 50 cubic metres or 10% of the original house volume, whichever is the greater.
Houses within a Conservation Area - 50 cubic metres or 10% of the original house volume, whichever is the greater.
All house (detached or semi-detached) - 70 cubic metres or 15% of the original house volume, whichever is the greater.
In all cases size is calculated from external measurement, and there is an upper limit of 115 cubic metres.
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).
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.
The design and materials to be used to build the proposed extension must be in keeping with the main house.
No part of the extension should be nearer to any road than the part of the original house nearest to the road.
No part of the extension should be higher than the highest part of the existing roof of the house.
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.
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).
In some cases other buildings on your property will be treated as extensions and will reduce the allowances for extending your house. If an extension to your home comes within 5 metres of another building belonging to the house (eg, garage, shed or greenhouse, etc.) that building will be treated as if it were part of the extension (see Illustration 1).
Any building added to your property which is more than 10 cubic metres in volume and which is within 5 metres of your house is treated as an extension and so reduces the allowances for further additions to the house without planning permission
(see Illustration 3).
Small shed (less than 10 cubic metres) is not treated as an extension but will reduce the allowance for further extension to the house (see Note 3).
A large shed/summerhouse/garage (greater than 10 cubic metres) is treated as an extension if within 5 metres of the house. If you live in a house which is a listed building or is in a Conservation Area, all additional buildings which are more than 10 cubic metres in volume, regardless of the distance from your house, are treated as extensions and reduce the allowance for extending without planning permission.
The figures given for maximum sizes in Rule A represent the total amounts which can be built without planning permission. If, for example, you are making an addition to the roof of your house and also extending the house, the volume of both structures added together must not exceed the relevant amount shown at Rule A.
If you live in a house which is a listed building, it is likely that you will need Listed Building Consent for the building operations covered in this booklet. Your local planning office will advise if such consent is required.