Do I need a party wall agreement for an extension?

Last updated: 30 July 2026

Usually, yes — and extensions are the awkward case, because one project can trigger up to three different notices at once: the digging, the new wall at the boundary, and the shared wall you might tie into. Here's which is which, in plain English.

The three ways an extension meets the Act

1. The dig (section 6). Extension foundations within 3 metres of your neighbour's building and deeper than their foundations need a notice — as does deeper work within 6 metres that cuts a line drawn down at 45 degrees from the bottom of their footings. Full explanation with a diagram: the 3-metre and 6-metre rules. Notice period: one month, and the notice must include plans and sections showing where and how deep.

2. The flank wall at the boundary (section 1). If your new side wall sits astride the line, you need your neighbour's written consent within 14 days of the notice — no consent, and the wall must be built wholly on your own land at your own expense. If it sits up to the line on your own land, you serve notice and can start a month later. And a wall set back from the boundary even by a couple of inches doesn't invoke section 1 at all — but run the digging test anyway, because the foundations usually still do.

3. Tying in (section 2). Cutting into a wall you share — to bond the new structure in, insert a beam or flashing — is work to a party structure. That's the two-month notice, the same one loft conversions use.

By extension type

The Act doesn't have rules for "side returns" or "two-storey rears" — it has the three tests above, and you run all three whatever you're building. But the tests land differently depending on the shape of the job, so here's how they usually fall.

Single-storey rear extension

The commonest case, and it's nearly always the dig that catches it, not the walls. New foundations along the back of a terrace or semi are routinely within 3 metres of next door and deeper than their footings, which is a section 6 notice. Add section 1 if a new flank wall goes up at the boundary, and section 2 if you tie into the shared wall or cut into it for a beam.

Side return and wraparound extensions

The one most likely to need all three. A side return by definition builds along the boundary, so a new wall at or up to the line of junction is section 1; the foundations for it sit right next to the neighbour, so section 6; and wrapping round to meet the existing rear wall often means cutting into the party wall, so section 2. Being narrow doesn't make it exempt — if anything it makes contact more likely.

Two-storey rear extension

Everything the single-storey version triggers, plus a higher chance of section 2. At first floor you're usually working against the party wall itself — tying in, cutting in for beams, sometimes raising it. Deeper foundations also make the section 6 tests easier to meet, not harder.

Conservatories and garden rooms

No special rule and no exemption — the same three tests apply. Because the foundations are often shallower, the section 6 answer genuinely can come out negative, but that depends on your neighbour's foundation depth as much as yours, and most people don't know it. If you don't, treat it as likely notifiable and confirm rather than assume. For smaller garden jobs — decking, a patio, solar panels — the usually-exempt list has the same tests applied.

Garage conversions and new garages

Depends entirely on whether the garage wall is shared or sits on the boundary, which is its own tangle — is my garage wall a party wall? works through it.

Detached houses

The word people search for, so let's use it plainly: a detached house usually has no party wall, and section 2 generally doesn't apply — the Act's section 2 needs the two properties to adjoin and be built on at the boundary. That is the one clean "no notice needed" path.

The catch that surprises people: section 6 doesn't care whether you're detached. It measures from your neighbour's building, not from the boundary, so a detached house on a tight plot digging foundations within 3 metres of next door's wall is squarely caught. Detached means "probably no section 1 or 2" — it does not mean "no notice".

The clocks, side by side

Each notice runs its own clock, every notice stays valid for 12 months, and every one must be served on every owner next door — both halves of a couple, and for flats the freeholder as well as the leaseholder.

So how long does the whole thing actually take?

Plan from the longest notice, not the shortest. If any part of the job is section 2 work, that's two months before you can start — serving the section 6 notice on the same day doesn't shorten it. Add time before that for drawings, since an excavation notice isn't valid without plans and sections.

If every neighbour consents, that's the whole delay. If anyone dissents, surveyors are appointed and an award has to be made first — and here's the part other sites gloss over: the Act sets no deadline for producing an award. There is no statutory timescale at all. Anyone quoting you a fixed number of weeks is describing trade convention, not law. Build your programme around the notice periods, which are certain, and treat the award stage as an unknown you want to avoid by getting consent.

What most people get wrong

Assuming "no shared wall, no notice". A detached-feeling side return still digs within 3 metres of next door. The clean no-notice extension does exist — well clear of the boundary, shallow foundations, nothing shared touched — but it's rarer than people hope, and if you're unsure about your neighbour's foundation depth, the honest answer is "likely notifiable — confirm with a surveyor". That's exactly how the free checker treats it: answer six questions and it names your notices, your clocks, and who to serve.

What if my neighbour refuses, or just ignores it?

Neither stops the extension. Your neighbour can dissent — they can't veto. Dissent moves the project into the surveyor process, which decides how and when the work happens, not whether. Silence isn't an escape either: after 14 days with no reply to a section 2 or section 6 notice, the Act treats it as dissent and the same process starts anyway. The full picture, from the neighbour's side of the fence, is in can I refuse a party wall agreement?

And if I just don't serve notice?

Failing to serve isn't a criminal offence — but it removes the Act's protections and leaves you exposed to the courts instead, where a neighbour can seek an injunction to stop work. There's no way to fix it afterwards, either: the Act contains no retrospective mechanism. What actually happens when no notice was served covers both sides of that.

What does an extension's party wall bill look like?

If everyone consents: as little as nothing, because serving the notices yourself is free. If it goes to surveyors, a single agreed surveyor on an extension averages around £2,025, and remember fees are per neighbour — a mid-terrace with attached neighbours on both sides pays twice. Who pays for a party wall surveyor sets out why that bill is usually yours, the London guide has the capital's numbers, and the costs page breaks it down by project.

Ready to serve? The official example letters are free — see the templates, explained — or write your own notice with the checklist that keeps it valid. The numbers live on what a party wall agreement costs.