Building an extension? When you need a party wall notice
Building an Extension? Here’s When the Party Wall Act Gets Involved
You’ve got planning permission, a builder, and a start date. Then someone mentions party wall notices, and suddenly everyone at the dinner party has an opinion. Let’s sort fact from folklore.
5 min read • Party Wall Online
The Party Wall etc. Act 1996 covers far more than the wall you share with next door. For extensions, there are three separate ways your project can fall under it, and most extensions built close to a neighbour trigger at least one.
The Three Triggers
- 1Work on the party wall itself. Cutting in beams, raising it, underpinning it, or tying your new extension into it. This needs two months’ notice.
- 2A new wall at the boundary. Building a wall up to, or astride, the line between the two properties. This needs one month’s notice.
- 3Digging near a neighbour’s building. Excavating for foundations within three metres of their structure, deeper than their foundations. This also needs one month’s notice, and it catches more people out than the other two combined.
The three-metre rule, in plain English
If any part of your dig comes within three metres of next door’s building and goes deeper than their foundations, you must serve notice. Most older houses sit on surprisingly shallow foundations, and modern extensions dig deeper, so in a terrace or semi this rule applies to almost every rear extension. A rarer six-metre version catches very deep work like basements.
You can build an extension without ever touching your neighbour’s wall and still be covered by the Act. What happens below ground doesn’t respect the fence line.
“But It’s Only a Small Extension”
The Act doesn’t have a size threshold. A modest single-storey kitchen extension and a double-storey wraparound are judged by the same tests: what are you doing to the shared wall, where is your new wall going, and how close and how deep are you digging.
The only extensions that genuinely escape are ones built well away from any boundary, typically on detached houses with generous gardens. If you’re reading this from a terrace, assume the Act applies until your drawings prove otherwise.
What Happens After the Notice
Your neighbour has 14 days to consent or dissent. Consent and you crack on. Dissent (or silence) and a surveyor prepares a party wall award setting out how the work proceeds and how their property is protected. It’s a well-trodden path, not a courtroom drama, and we’ve covered the details in our guides to how long the process takes and what it costs.
Your Pre-Build Checklist
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Check your drawings against the three triggers Distances to the boundary, depth of the new foundations, and anything touching the shared wall. Your architect or a party wall surveyor can confirm in minutes.
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Work out who needs a notice It might be more people than you think: both neighbours on a mid-terrace, and both freeholders and long leaseholders where flats are involved.
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Have the conversation before the letter Show them the plans over the fence. Neighbours who feel informed consent quickly; neighbours who feel ambushed reach for a surveyor.
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Serve notices as early as you can Notices stay valid for twelve months, and the statutory notice periods run whether or not your builder is ready. Early costs nothing; late costs weeks.
An extension is stressful enough without a last-minute legal surprise. Ten minutes with your drawings now saves a month on site later.
Not sure if your extension needs a notice?
Send Party Wall Online your drawings and we’ll tell you exactly which notices you need, if any. RICS-qualified surveyors, fixed fees, England and Wales. The first conversation is always free.
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