Do I need a party wall agreement for a basement conversion?

Almost certainly, and usually more than one notice. Excavating for or under a basement near a neighbouring building engages section 6, which needs one month's notice with plans and sections. If the work also underpins or otherwise affects a wall you share, that is section 2 work needing two months' notice. Basements are the project type where professional advice is worth taking from day one.

Can my neighbour stop my basement conversion?

They cannot veto it. They can dissent, which moves the project into the Act's dispute procedure where surveyors decide how and when the work is carried out, what protections apply, and how the properties are monitored. On a basement that process is doing real work rather than being a formality, so expect it to take time and to shape the method.

Can I make my neighbour underpin my foundations before they dig?

Where section 6 applies, the building owner must, if required by the adjoining owner, underpin or otherwise strengthen or safeguard the adjoining owner's foundations — but only so far as may be necessary. It is not automatic and it is not unlimited. What is necessary is a question for the appointed surveyors, and who ultimately pays is genuinely unresolved in the Act, so it is settled in the award.

My neighbour is digging a basement — what protection do I have?

You are entitled to a section 6 notice at least one month before digging, including plans and sections showing where and how deep, and stating whether they propose to underpin your foundations. You can dissent and have a surveyor act for you, normally at your neighbour's expense. You can ask for a schedule of condition recording your property beforehand, and you can serve a notice requiring security for expenses before work starts.

Basement conversions and underpinning: the party wall rules

Last updated: 31 July 2026

Basements are the party wall Act's hardest case. They routinely trigger two different notices at once, they're the one project where the neighbour has a right most people have never heard of, and they're where the Act's silences bite hardest. If you're doing one — or living next to one — this is the shape of it.

Why basements trigger more than one notice

Most projects engage one part of the Act. A basement usually engages two, because the work has two distinct effects on the neighbour.

The dig — section 6

Excavation near a neighbouring building is caught by either of two independent tests: digging within 3 metres of any part of their building and going deeper than the bottom of their foundations, or within 6 metres and deep enough to cut a line drawn down at 45° from the bottom of those foundations.

Basements meet these tests more easily than any other project, because the 6-metre rule exists for exactly this kind of depth. Notice period: one month — and it is not valid without plans and sections showing where and how deep. The notice must also state whether you propose to underpin or otherwise strengthen the neighbour's foundations. Full detail: the 3-metre and 6-metre rules.

The shared wall — section 2

If the work underpins, thickens or otherwise alters a wall you share, that's a separate right under section 2 and needs a party structure notice, two months ahead. Underpinning a party wall is expressly one of the section 2 rights.

Two notices, two clocks. Serving the one-month notice on the same day doesn't shorten the two-month one, so plan from the longer period — and remember every notice must go to every owner next door, which in a converted terrace means leaseholders and the freeholder.

What we won't tell you

You'll find pages quoting typical basement depths, typical Victorian foundation depths, and rules of thumb about which soils need what. We don't publish those, because the Act contains no depth figures at all and neither does the government's guidance — they're engineering judgements dressed up as legal thresholds.

The Act gives you tests, not numbers: distance from the neighbour's building, and depth relative to their foundations — which is the figure almost nobody knows. If you don't know how deep next door's footings go, the honest answer isn't a table on a website, it's that the job is likely notifiable and someone should establish it properly.

The right your neighbour has that nobody mentions

This is the most important thing on this page for both sides, and it's routinely left out.

Where section 6 applies, the building owner must — if required by the adjoining owner — underpin or otherwise strengthen or safeguard the foundations of that neighbour's building, so far as may be necessary.

Both halves of that sentence matter. It isn't automatic: the neighbour has to actually require it. And it isn't unlimited: the obligation runs only as far as necessary. What "necessary" means, and how far the work goes, is a question for the appointed surveyors — we're not going to quantify it, and neither does the Act.

Who pays for it is genuinely unresolved

Here the statute contradicts itself, and we'd rather say so than pick a side. The excavation section says the building owner does this work at his own expense. A different section says that where works are carried out in pursuance of a requirement made by the adjoining owner, the adjoining owner defrays the expenses.

Nothing reconciles them. So treat cost as something the award decides — the Act expressly sends disputes about responsibility for expenses to the surveyors — and be sceptical of any page that answers it confidently in either direction.

How to make the request

The Act prescribes no form, no wording and no deadline for requiring underpinning, and it is not a counter-notice — counter-notices exist only in response to a party structure notice and carry their own one-month clock.

Practically: put it in writing, early, and keep a copy. Raise it with your surveyor as soon as you dissent rather than once the diggers are on site, because the machinery for resolving it is the award, and the award is easier to shape before work than after.

If you're the neighbour: what protects you

A proper notice. One month, with plans and sections, and a statement of whether they propose to underpin. A section 6 notice without drawings may be defective — raise that before responding, not after.
A surveyor at their expense. Dissent and appoint your own, and in the standard case the building owner pays your reasonable fees. On a basement this is rarely the moment to be stoic about it.
A schedule of condition. Dated written and photographic record of your property before anything starts. Not required by the Act — the government's guidance says so, then says it's most prudent anyway — and on a basement it's the difference between a five-minute comparison and a year-long argument.
The underpinning right above.
Security for expenses. See below.

Security for expenses — the basement lever

If what worries you is a deep excavation being abandoned half-finished next to your house, this is the provision for it. You can serve a notice requiring the building owner to provide security before they begin — and it applies even though the digging is entirely on their own land, which the courts have confirmed.

Two honest caveats. The Act says nothing about how much, what form it takes, or how and when it comes back; that's agreed between the owners or settled in the award. And there's a catch: serving that notice entitles the building owner to ask you for security in return, and if you don't meet that within a month, your own request falls away. It doesn't affect your dissent, your right to an award, or your right to compensation for damage — but the request itself dies. Raise it with your surveyor before serving.

Timings, and the part you can't plan

The notice periods are certain: two months for section 2, one month for section 6, both running from service, and every notice lapsing after 12 months if work hasn't begun.

The award is not. The Act sets no deadline for producing one — there's no statutory timescale between a dispute arising and an award being made. On a straightforward loft that's a footnote. On a basement, where the surveyors have real engineering to consider and may want monitoring regimes and method statements in the award, it's the single biggest unknown in your programme. Anyone quoting you a fixed number of weeks is describing custom.

Which is the practical argument for serving early, explaining the work properly, and making consent as easy as you can — though on a basement, dissent is often the sensible choice for the neighbour, and worth expecting rather than resenting.

What it costs

Basements carry the highest party wall costs of any project type: published averages put a single agreed surveyor around £3,375, against roughly £1,575 for a loft. In London, £750–£1,800 per neighbour is commonly quoted, and basement projects frequently involve several neighbours at once — fees are per neighbour, not per project.

Serving the notices yourself is still free, and a consenting neighbour still costs nothing further. But this is the project type where DIY is most often false economy: complex engineering, near-certain dissent, and a lot riding on the method the award sets. What a party wall agreement costs has the full picture.

Planning and Building Regs are separate again

Basement projects usually need planning permission and certainly need Building Regulations approval, and some boroughs have their own basement policies. None of that is the Party Wall Act. Planning consent doesn't notify your neighbour, and a party wall award grants no planning permission. Three systems, three approvers, each satisfied on its own terms.

Not sure which notices your project needs? The free checker runs both tests and names them in about 90 seconds — and treats an unknown foundation depth as likely notifiable rather than guessing in your favour. For the whole process end to end, see the Act explained, and for what the surveyors eventually produce, the party wall award.