This is the single most common question we get asked, usually about ten seconds into the first phone call. A great many single storey rear extensions never need a planning application at all.
Permitted development, in plain English
Permitted development is a set of national rules that let you build certain things without applying for permission. It is not a loophole and it is not a grey area. It is a defined allowance, and if you build inside it, you are allowed to build.
The limits cover how far you project from the original rear wall, the height at the eaves and at the ridge, how close you build to a boundary, the materials you use, and how much of the original garden ends up built on.
The trap almost everybody falls into
The rules measure from the original house, not the house as it stands today. If a previous owner built a conservatory or a small extension in 1998, that has already used up part of your allowance even though you never saw it happen.
This catches people out constantly. Check what has been added before, not just what is there now.
When you will almost certainly need permission
- Two storey extensions, in nearly all cases
- Most wraparounds, because of the combined projection
- Anything forward of the principal elevation, meaning in front of the house
- Flats and maisonettes, which have no permitted development rights at all
- Listed buildings, where you also need listed building consent
- Conservation areas and designated land, where the allowances are much tighter
Building regulations are a separate thing
People conflate these two constantly. Planning permission is about whether you are allowed to build the thing. Building regulations are about whether it is built safely and properly, covering structure, insulation, fire safety, ventilation and drainage.
Almost every extension needs building control sign off even when it needs no planning permission whatsoever. These are different departments, different applications and different purposes.
The lawful development certificate
If you are clearly inside the permitted development limits, you can simply build. But if you are marginal, or if you want certainty, apply for a lawful development certificate. That is your council confirming in writing that what you are proposing is allowed.
It costs a fraction of a full planning application, and it is the document your buyer's solicitor will ask for when you come to sell in ten years time. Not having it can hold up a sale. We recommend it whenever there is any doubt at all.
What about the neighbours?
There is also the party wall side, which is separate again from both planning and building control. If you are excavating near a shared boundary or building on the line of it, you may need to serve party wall notices on your neighbours. It is a legal process with defined notice periods, and skipping it is how neighbours end up with solicitors involved.
What we do about all of this
We check permitted development against the original house at the first visit, before you have spent anything at all. If you are inside, we say so. If you are marginal, we recommend the certificate. If you need full permission, we tell you honestly what the realistic timescale is rather than pretending it will be quick.
This is general guidance, not a formal planning opinion. Rules change and every plot is different, which is exactly why yours needs checking properly.
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