Many single storey rear and side extensions to houses in England can be built without a full planning application under permitted development rights, as long as they stay within set limits on depth, height, width and materials. Two storey extensions, extensions facing a road, work on flats or maisonettes, and homes in conservation areas or with removed rights usually need planning permission. Almost every extension, with or without planning, needs Building Regulations approval. If in doubt, a lawful development certificate from your council confirms the position in writing.
What permitted development allows
Permitted development is a national set of rules that lets householders make certain changes without applying for planning permission. For extensions, the main limits in England are summarised below. They are a guide only; the official government technical guidance has the full conditions and exceptions.
- A single storey rear extension can typically go up to 3 metres deep on an attached house or 4 metres on a detached house.
- The extension should be no higher than 4 metres, and eaves within 2 metres of a boundary should be no higher than 3 metres.
- Side extensions should be single storey, no more than 4 metres high and no wider than half the width of the original house.
- Nothing can extend beyond the wall facing a highway at the front or side of the house.
- Materials should be similar in appearance to the existing house.
- Extensions and outbuildings together must not cover more than half the land around the original house.
Larger rear extensions and prior approval
There is also a larger home extension scheme that can allow single storey rear extensions up to 6 metres deep on an attached house or 8 metres on a detached house. This is not automatic: you must notify the council first, and neighbours are consulted. If they raise objections the council considers the impact on their amenity before deciding. Work should not start until prior approval is confirmed.
When you will need planning permission
Full householder planning permission is usually needed when a project falls outside those limits. Common examples are two storey side extensions, most two storey rear extensions close to a boundary, front extensions, and anything on a flat or maisonette, which do not have householder permitted development rights. Listed buildings need listed building consent for works affecting their character, and homes in conservation areas have tighter rules, particularly on side extensions and cladding.
Some homes have had their permitted development rights removed by a planning condition or an Article 4 direction, especially on newer estates. It is worth checking your property's planning history before assuming you can build without an application.
Building Regulations are separate
Planning decides whether you may build; Building Regulations decide how it must be built. Foundations, structure, fire safety, insulation, ventilation, drainage and electrics all need to meet the regulations, and a building control body inspects the work at key stages. At the end you should receive a completion certificate, which you will want when you sell the house.
Why a lawful development certificate helps
If your extension is permitted development, you do not have to apply for anything. Even so, many homeowners apply for a lawful development certificate. It is formal confirmation from the council that the work is lawful, which can prevent problems if a neighbour questions it or when a buyer's solicitor asks for evidence years later.
Neighbours and party walls
Building on or near a shared boundary can bring the Party Wall etc. Act 1996 into play. If you are building on the boundary line, cutting into a shared wall, or digging foundations within a certain distance of a neighbour's, you normally need to serve notice before work starts. Talking to neighbours early tends to make the whole process smoother.
Checking with your council in Northamptonshire
Since 2021, Northamptonshire has two unitary councils. Northampton, Towcester and the south and west of the county fall under West Northamptonshire Council; Kettering, Wellingborough and the north and east are covered by North Northamptonshire Council. Your council's planning team or the national Planning Portal can confirm what applies to your property.
Superior Build & Landscape builds home extensions for homeowners across Northamptonshire. We are not planning consultants, but George is happy to talk through your ideas and how the build side works once your drawings and approvals are in place.
Frequently asked questions
What size extension can I build without planning permission?
Under permitted development in England, typically a single storey rear extension up to 3 metres deep on an attached house or 4 metres on a detached house, no higher than 4 metres, subject to other conditions. Larger rear extensions may be possible through the prior approval process.
Do I need planning permission for a single storey extension?
Often not, if it stays within permitted development limits and your home still has those rights. Front extensions, flats, listed buildings and some conservation area homes are exceptions.
Do I need Building Regulations if I don't need planning?
Yes. Almost all extensions need Building Regulations approval even when planning permission is not required.
Is a lawful development certificate compulsory?
No, but it gives written proof that the extension is lawful, which is useful for peace of mind and when selling your home.