Rear extensions are the most common domestic building project in the UK — and they're also one of the most common triggers for the Party Wall etc. Act 1996. Yet thousands of homeowners start works every year without having served the correct notices, only to discover the problem mid-build.
This guide covers when a party wall notice is required for an extension, what type of notice you need, and how to get the process right.
Does every extension need a party wall notice?
No — but most do. Whether you need one depends on the relationship between your extension and the adjoining property, specifically:
- Whether your extension includes or affects a party wall (the shared wall with your neighbour)
- Whether your extension's foundations will be dug within 3 metres (or 6 metres in some cases) of your neighbour's foundations
Single-storey rear extensions on a semi-detached or terraced property will almost always require at least one notice — typically a Section 6 notice for the excavation, and sometimes a Section 3 notice if the extension abuts or uses the party wall. Even if the extension is built entirely on your own land, if the foundations are dug within 3 metres of your neighbour's foundations at the same depth or deeper, the Act applies.
Two-storey rear extensions that tie into the party wall for structural support will need a Section 3 notice, in addition to any Section 6 notice for excavations.
Side extensions that build up to or on the boundary line require a Section 1 (line of junction) notice where the new wall is built astride the boundary, or where you build a new wall on your own land up to the boundary.
Which type of notice does your extension need?
Section 3 — Party Structure Notice
Required when your extension involves works to or affecting an existing party wall or party structure. Common for:
- Building off the party wall (using it to support the new structure)
- Cutting into the party wall to insert padstones, beams, or flashing
- Structural work to the party wall as part of the extension build
Notice period: two months before works start. See our full Section 3 guide.
Section 6 — Adjacent Excavation Notice
Required when you're digging foundations within 3 metres of your neighbour's building at the same depth or deeper, or within 6 metres if your foundations will cut a line drawn at 45 degrees from the base of their foundation.
Almost all rear extension foundations will trigger this. The relevant distance is measured from your neighbour's building — not from the boundary.
Notice period: one month before works start. See our full Section 6 guide.
Section 1 — Line of Junction Notice
Required if you're building a new wall on or up to the boundary line. Relevant for side extensions building to the boundary rather than setting back from it.
Notice period: one month before works start. See our full Section 1 guide.
Many extensions trigger more than one notice type. A standard single-storey rear extension on a semi-detached property might need both a Section 3 and a Section 6, served on the same neighbour at the same time.
When should you serve the notice?
The notice period starts from the date the notice is served — not when your planning application is submitted, not when your building regulations application is approved, and not when your contractor is ready to start.
This catches many homeowners out. Planning permission can take eight to twelve weeks. By the time you have approval and a start date, your contractor may be available within six weeks — which is too late if you haven't served your party wall notice yet.
The practical answer: start the party wall process at the same time as your planning application. You don't need planning permission in place before serving a party wall notice. The two processes can run in parallel.
Do you need to notify both neighbours?
For a typical semi-detached rear extension, you'll usually only need to notify the neighbour on the side where works are taking place. But if your foundations are within 3–6 metres of properties to the rear of your garden, you may also need to notify those owners.
In a terraced property, you may need to notify both side neighbours depending on the nature of the works.
Each notice is served separately. Each adjoining owner can respond independently — one may consent while the other dissents.
What makes a valid party wall notice for an extension?
The notice must include:
- Your full name and address
- Your neighbour's full name and address (and mortgagees if they have a financial interest)
- The site address
- The type of notice (Section 3, 6, or 1)
- A clear description of the works — enough detail that your neighbour and any surveyor they appoint can understand exactly what's proposed
- For Section 6: drawings showing the location and depth of excavations
- The proposed start date
- Your signature and date
The description of works is where most self-prepared notices fall short. "Rear extension" is not a sufficient works description. Something like "construction of single-storey rear extension; strip foundations at approximately 900mm depth within 2.5 metres of [adjoining owner's] building; new extension to abut and be bonded to existing party wall" is the kind of detail required.
For Section 6 specifically, drawings must accompany the notice. This isn't optional — it's a statutory requirement.
What happens next?
Your neighbour has 14 days to respond. If they consent, you can proceed after the relevant notice period has run.
If they dissent — or fail to respond at all, which counts as a deemed dissent — the dispute resolution process kicks in and surveyors are appointed. This doesn't block your project; it creates the legal framework under which it proceeds. A Party Wall Award sets out the conditions for the works, including any special measures to protect the adjoining property.
The consent rate across all party wall notices is around 45%. A well-prepared notice, served at the right time with the right detail, gives you the best chance of a straightforward consent.
What about the neighbour at the rear?
If your extension stops well short of the rear boundary and its foundations are more than 3 metres from any adjoining structure, the rear owner probably doesn't need to be notified. But if you're building close to the back garden wall, or if there's a neighbouring property close behind, check the distances carefully. An overlooked Section 6 notice is a statutory breach — and your contractor can't claim they didn't know.
Start the process now
The sooner you serve your party wall notice, the sooner the clock starts running. Extensions that run into party wall delays almost always started the process too late. Not sure whether you need one? Take our free eligibility checker first.
Generate a legally correct extension party wall notice in minutes, from £30 inc VAT.