For a single-storey side extension

The following permitted development restrictions apply to all extensions and alterations to a dwellinghouse:

Residential properties created as a result of prior approval (e.g. change of use from office to residential) do not benefit from householder permitted development rights. All new extensions will therefore require planning permission.
No more than 50% of the curtilage of the dwelling should be covered by buildings or extensions (excluding the original dwelling).
No extension should exceed the height of the highest part of the roof of the existing dwelling.
The eaves height of the extension should not exceed the eaves height on the existing dwelling.
If the extension is within 2 metres of your boundaries, the eaves height should not exceed 3 metres high;
The extension should not include or incorporate a veranda, balcony, raised platform, microwave antenna, chimney, flue or soil and vent pipe or alterations to any part of the roof of the dwelling.
If the property is in on Article 2(3) land, i.e. situated within a National Park, an Area of Outstanding Natural Beauty, a Conservation Area, an area designated under s41(3) of the Wildlife and Countryside Act 1981, the Norfolk Broads or a World Heritage Site, no part of the original or extended house should be clad in stone, artificial stone, pebble dash, render, timber, plastic or tiles.
Unless it is a conservatory, the extension should be built using external materials similar in appearance to those on the existing house;

Single-storey side extensions will not require planning permission provided they also comply with the following criteria:

The proposed side extension would not be located between the house and a highway, i.e. if the house is on a corner plot and an extension is proposed on the side facing the highway then planning permission is required.
The extension would be single storey in height and no more than 4 metres high.
The extension would exceed half the width of the original dwelling. On a detached house, provided the plot can accommodate them, it is possible to build a “half-width” single-storey extension on each side