The short answer is that many single storey extensions do not need planning permission, because they fall under permitted development rights. The longer answer depends on the size of the extension, the type of property you own and, in London especially, which borough you live in. Here is how to work out where your project stands before you spend money on drawings.
Permitted development rights let you extend a house without a planning application, provided the extension stays within set limits. For a single storey rear extension, that generally means projecting no more than 3 metres from the original rear wall of a terraced or semi detached house, or 4 metres for a detached house. The extension must be no taller than 4 metres overall, and if it comes within 2 metres of a boundary the eaves must not exceed 3 metres.
There is also a larger home extension route, sometimes called prior approval, which allows up to 6 metres on a terraced or semi detached house and 8 metres on a detached one. This is not automatic. You notify the council, they consult your neighbours for 21 days, and if nobody objects with valid planning grounds you usually get the go ahead within 42 days.
Permitted development rights apply to houses, not flats or maisonettes. A large share of London properties are flats, including many converted Victorian terraces, and if yours is one of them you will need a full planning application for any extension, however modest.
Conservation areas are the other common catch. Boroughs such as Islington, Camden, Hackney and Richmond have extensive conservation areas where side extensions lose permitted development rights and the larger home extension route is unavailable. Some London councils also use Article 4 directions to remove permitted development rights street by street, so two identical houses a few roads apart can face completely different rules. A quick call to your borough planning department, or a search on their online policy map, settles this in minutes.
The size limits are measured from the original rear wall, which means the house as it stood when first built or as it was on 1 July 1948 for older properties. This trips up a lot of London homeowners, because so much of the housing stock has already been altered.
If a previous owner added a rear addition or infilled a side return, that existing structure counts towards your allowance. A 1980s lean to that projects 2 metres leaves you far less permitted development room than you might assume. Checking old drawings, the council planning history and your property deeds before designing anything saves redesign costs later.
Even if your extension qualifies as permitted development, we would always recommend applying for a lawful development certificate. It costs 129 pounds in England, takes up to 8 weeks, and gives you formal proof that the work was lawful, which solicitors increasingly ask for when you sell. Without it, you are relying on your own interpretation of the rules.
If you do need full planning permission, a householder application costs 258 pounds plus preparation of drawings, and councils aim to decide within 8 weeks. Separately, building regulations approval is required for every extension regardless of planning status, covering structure, insulation, drainage and glazing. And if you are building on or near a shared boundary, which is the norm for London terraces, the Party Wall Act means serving formal notice on your neighbours at least 2 months before starting structural work.
A householder planning application should be decided within 8 weeks of validation, though busy London boroughs sometimes take longer. The prior approval route for larger extensions has a fixed 42 day deadline.
No. If the extension genuinely meets the permitted development criteria, neighbour objections carry no weight. Under the larger home extension scheme neighbours are consulted, but the council can only refuse on the impact to their amenity, not simply because they object.
Often not, as infilling a side return usually qualifies as permitted development on a house outside a conservation area. Because side returns sit tight to the boundary, the 3 metre eaves limit and party wall notices almost always apply, so get the design checked before committing.