In short: you usually need a party wall agreement if your extension is built on or up to a boundary, cuts into a shared wall, or has foundations within 3 metres of a neighbour's building (sometimes 6m if you dig deeper than theirs). Serve notice at least two months before you start. If the neighbour consents there is no surveyor to pay; if they dissent, a surveyor prepares an award, typically £1,000–£2,000, which you pay as the building owner. It comes from the Party Wall etc. Act 1996.
A party wall agreement is one of the most misunderstood parts of building an extension — and getting it wrong can stall the whole job. This is a plain-English guide to when you need one, what it costs and how it affects your timeline. It is general guidance, not legal advice.
What a party wall agreement is
The Party Wall etc. Act 1996 is there to protect both you and your neighbour when building work happens near a shared boundary. It sets out a formal way to give notice, record the condition of the neighbour's property, and agree how the work will be done. The “agreement” (formally called an award) is the document that results if a surveyor becomes involved.
When you need one for an extension
You usually need to serve a party wall notice if your extension involves any of:
- Building on or up to the boundary line with the neighbour.
- Cutting into or building off a shared (party) wall — for example inserting a beam.
- Digging foundations within 3 metres of a neighbour's building, or within 6 metres if your foundations go deeper than theirs.
Side-return, wrap-around and many rear extensions trigger the Act because they sit close to a boundary. A small extension well away from any boundary, with shallow foundations, may not.
How to serve notice, and the timeline
You (or a surveyor on your behalf) serve written notice on the affected neighbour. The neighbour then has 14 days to respond. They can:
- Consent — in writing, and you can usually proceed without a surveyor.
- Dissent — and a surveyor (or one each) is appointed to draw up a party wall award.
- Not respond — after 14 days this counts as a dissent, and the surveyor route follows.
The formal notice period is two months for most works before you can start. That is why serving notice early — well before you want to break ground — is the best way to keep your build on schedule.
What it costs and who pays
| Situation | Typical cost |
|---|---|
| Neighbour consents in writing | Often no surveyor fee |
| Single “agreed surveyor” (extension) | £1,000–£2,000 |
| A surveyor each side | Can roughly double |
As the building owner you normally pay the fees, including the neighbour's reasonable surveyor costs. Adjoining owners' surveyors often charge £150–£250 an hour. Budget for it from the outset so it is not a surprise.
How it affects your build
The party wall process runs alongside your planning and building-regulations work, not after it, so the smart move is to sort it early. Good neighbour relations help enormously — a friendly conversation before the formal notice lands often means a quick written consent and no surveyor at all. Our guide on the order to do things and who to contact first shows where this fits in the sequence.
Where we fit in
We are the builder, not a party wall surveyor — but we work to the agreed award and coordinate with your surveyor so the brickwork is done exactly as recorded. If your extension is likely to affect a boundary, see our side-return extension cost and extension pages.
This is general guidance on the Party Wall etc. Act 1996, not legal advice. For your specific situation, speak to a party wall surveyor.
Planning an extension near a boundary?
If you are planning an extension in Hampshire, Berkshire or Surrey and want a builder who works cleanly to a party wall award, send us the details and we will come and look.
Frequently asked questions
Do I need a party wall agreement for an extension?
Often, yes. You usually need to serve a party wall notice if your extension involves building on or up to the boundary, cutting into a shared wall, or digging foundations within 3 metres of a neighbour's building (and sometimes within 6 metres if you dig deeper than their foundations). If none of those apply, you may not need one. The rules come from the Party Wall etc. Act 1996.
How much does a party wall surveyor cost?
For an extension, a party wall agreement commonly costs around £1,000 to £2,000 where a single surveyor acts, and more if each side appoints their own surveyor. Adjoining owners' surveyors often charge £150 to £250 an hour. Basement and more complex jobs cost more. If a neighbour simply consents in writing, you may avoid surveyor fees altogether.
Who pays for the party wall surveyor?
As the person doing the building work, you (the building owner) normally pay the party wall surveyor fees, including your neighbour's reasonable surveyor costs if they appoint their own. This is because the work is for your benefit. It is worth budgeting for it from the start rather than being surprised by it.
How long does a party wall agreement take?
Allow at least two months. The formal notice period is two months for most party wall works before you can start, and if the neighbour dissents it then takes time for surveyors to prepare the award. Serving notice early, well before you want to break ground, is the single best way to avoid your build being delayed.
What happens if my neighbour ignores the party wall notice?
If a neighbour does not respond within 14 days, they are treated as having dissented, and the matter goes to surveyors to resolve with a party wall award. It does not stop your project; it simply means the formal surveyor route is followed. Ignoring the process yourself, though, can lead to an injunction, so it is not worth skipping.
Can I build an extension without a party wall agreement?
Only if the work does not fall under the Party Wall Act, for example a small extension well away from any boundary with shallow foundations. If the work does fall under the Act and you proceed without serving notice, a neighbour can seek an injunction to stop the work, which is far more costly and stressful than doing it correctly.