Extending your house:  Building regulations for extentions (General guidance);

You need to apply for planning permission to extend or add to your house in the following circumstances.  

You want to build an addition, which would be nearer to any highway than the nearest part of the original house unless there would be at least 20 metres between your house (when extended) and the highway. The term highway includes all public roads, footpaths, bridle ways and byways. 

More than half of the area of land surrounding the original house would be covered by additions or other buildings. 

The term “original house” means the house as it was first built, or as it stood on 1st July 1948 if it was built before that date.(You may not have built an extension, but the previous owners may have). 

If none of the above rules apply to your proposed extension you may need to check the height and volume limits. 

You will need permission if the extension is higher than the highest part of the roof of the original house, or any part of the extension is more than 4 metres high and is within 2 metres of the boundary of your property. (loft conversions and dormers have separate rules.

You will need permission if:

For a terraced house, including an end of terrace, or any house in the restricted areas described above - 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. - In all cases, if the volume of the original house would be increased by more than 115 cubic metres. 

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

If an extension comes within 5 metres of another building belonging to your house, the volume of that building counts against the allowance for additions and extensions.

Any building which has been added to your property and which is more than 10 cubic metres in volume and which is within 5 meters of your house, is treated as an extension of the house and so reduces the allowance for further extensions .

If you live in one of the restricted areas all additional buildings which are more than 10 cubic metres in volume wherever they are in relation to the house are treated as extensions to the house and reduce the allowance for further extensions.