A single storey side extension can usually be built without planning permission in England if it stays within permitted development: no more than 4 metres high, no wider than half the width of the original house, and with eaves no higher than 3 metres if it is within 2 metres of a boundary. A side extension of more than one storey always needs planning permission, and so does any side extension on designated (article 2(3)) land, which includes conservation areas, National Parks, the Broads, Areas of Outstanding Natural Beauty and World Heritage Sites.
Permitted development rules for a side extension
Permitted development rights for extensions come from Class A of Part 1, Schedule 2 of the General Permitted Development Order 2015. The Planning Portal summarises them, and the GOV.UK technical guidance explains how they are measured. For a side extension, these are the limits that matter:
- Single storey only.
- No more than 4 metres high.
- No wider than half the width of the original house.
- Eaves no higher than the eaves of the existing house, and no higher than 3 metres if any part of the extension is within 2 metres of a boundary.
- No higher than the highest part of the existing roof.
- Not forward of the principal elevation, or of a side elevation that fronts a highway.
- Extensions and other buildings must not cover more than half the land around the original house.
- Exterior materials of a similar appearance to the existing house.
- No verandas, balconies or raised platforms.
These rights apply to houses only. Flats and maisonettes do not have them, and they can be removed by an Article 4 direction or restricted by a condition on an earlier planning permission. The limits also apply to the total enlargement, so an earlier extension that the new one joins on to counts towards the limits.
What counts as the original house
The technical guidance defines the original house as it stood on 1 July 1948, or as it was first built if it was built after that date. If a previous owner has already extended the house, that extension is not part of the original house, and if your new extension joins on to it, the limits are applied to the old and new work together.
How the half-width rule is measured
The guidance says the width of the original house is measured at its widest point. It also defines a side wall as any wall that cannot be identified as a front wall or a rear wall, so many houses have more than two side walls. Its worked example shows a side extension followed later by a rear extension joined to it: if the two together are wider than half the width of the original house, the later extension needs planning permission, even though the first one complied on its own.
When a side extension needs planning permission
You will need a householder planning application for a side extension if any of these apply:
- It has more than one storey. The Planning Portal says any side extension of more than one storey needs householder planning permission.
- The house is on designated land. In conservation areas, National Parks, the Broads, Areas of Outstanding Natural Beauty and World Heritage Sites, extensions beyond any side wall of the original house are not permitted development.
- It breaks one of the limits above, for example it is wider than half the original house or taller than 4 metres.
- Permitted development rights have been removed, or the property is a flat or maisonette.
Two-storey side extensions are a request we often see, and they always go through a planning application. Our guide to a householder planning application explains how that process works.
What councils tend to look for
Each council sets its own design standards, so check your council's guidance. Two published guides show the kind of thing councils look for. The South Bucks House Extensions Design Guide asks for two-storey side extensions to be set at least one metre from the side boundary, so that a terracing effect is not produced, although it says this could be waived next to open land or a public highway that is likely to stay permanently open. It also says that where the house has a pitched roof, a two-storey extension should have one too, at the same or a similar angle.
Reigate and Banstead's guidance on two-storey side extensions (March 2004) says a one metre gap to the boundary will normally be required. It also explains that setting the extension well back from the main front wall, which lowers its ridge line, helps it read as subservient to the house in the street scene.
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Details that often change the answer
Corner plots
If the side of your house faces a road, the side elevation may count as fronting a highway, and an extension beyond it is not permitted development. The technical guidance says an elevation at more than 45 degrees to the highway will not normally be treated as fronting it, so the angle of the house to the road matters.
Wrap-around extensions
An extension that fills the space beside the house and also projects past the rear wall has to meet both the side and the rear limits. If you are thinking about filling in the side of a terraced house, our post on the side return extension covers that case, and the prior approval for a larger home extension post explains the longer rear extension route.
Height near the boundary
Many side extensions sit close to the boundary, so the 3 metre eaves limit within 2 metres of a boundary is often the one that decides the design. The guidance measures eaves from ground level at the base of the outside wall to the point where the wall would meet the roof, ignoring overhangs and parapets.
How we check a side extension
The route depends on the width of the house and the position of the boundary, so before advising on it we ask for a few things: a photo of the front of the house from the street, a photo of the side where the extension would go, a wider photo showing the boundary and the neighbouring house, and the rough width of the house and of the extension you have in mind, plus any existing floor plans.
Getting certainty before you build
If your side extension fits within permitted development, you do not have to apply for anything, but you can apply for a lawful development certificate if you want certainty. GOV.UK explains that a certificate confirms proposed works would be lawful, and that once granted, they are presumed lawful unless there is a material change before work starts. A certificate covers planning law only and does not remove the need to comply with other legal requirements such as the Building Regulations.
Party wall notices
If a side extension is built on or at the boundary, or its foundations are dug below and near to the neighbour's foundation level, GOV.UK says you must tell your neighbour, and give notice between 2 months and a year before you plan to start.
Frequently asked questions
Do I need planning permission for a side extension?
Not always. A single storey side extension that is no more than 4 metres high, no wider than half the original house and within the other Class A limits is usually permitted development. Two-storey side extensions, and side extensions on designated land, need planning permission.
How wide can a side extension be without planning permission?
No wider than half the width of the original house, measured at its widest point. Any earlier extension it joins counts towards that width.
How close to the boundary can I build a side extension?
Permitted development does not set a minimum gap, but if any part of the extension is within 2 metres of the boundary, the eaves cannot be higher than 3 metres. For two-storey side extensions, councils such as Buckinghamshire (South Bucks) and Reigate and Banstead expect a gap of at least one metre to the boundary.
Can I build a two storey side extension under permitted development?
No. All side extensions of more than one storey need planning permission.
Talk it through with us
If you are not sure whether your side extension fits within permitted development, or what your council is likely to look for, we are happy to take a look. Book a free advice call and we will talk you through the likely route for your house.