The short answer
On a London terrace, almost always. Digging foundations within 3 metres of a neighbour's house, or building on or against the boundary wall, falls under the Party Wall etc. Act 1996. You serve notice; the neighbour either agrees in writing or a surveyor draws up an award.
“Party wall agreement” is the everyday name for one of two things: the neighbour’s written consent to your notice, or, if they do not consent, a party wall award drawn up by surveyors. Either one lets the work go ahead lawfully.
Which parts of an extension trigger the Act
The Party Wall etc. Act 1996 covers three kinds of work, and a typical London extension can involve all three.
- Work to an existing party wall. Cutting in a steel beam, inserting flashing, or raising the shared wall between two back additions. Notice: at least two months.
- A new wall on the boundary. Building the side wall of a side return on the line between the two properties. Notice: at least one month.
- Excavation near a neighbour’s building. Digging foundations within 3 metres of their building and deeper than its foundations, or within 6 metres where a 45 degree line from the bottom of your trench would pass below theirs. Notice: at least one month.
Victorian houses often have shallow footings, so new extension foundations are usually deeper. On a mid-terrace, that brings in both neighbours.
What happens after the notice
The neighbour has 14 days to reply. They can consent, in which case you can start once the notice period runs out. Or they can dissent, or simply not reply, and the Act treats that as a dispute. Then a surveyor, or two surveyors, prepare an award setting out how the work will be done, when, and what happens if damage is caused.
You cannot serve notice after work has started. Serve it once the design is settled and before the foundations are dug.
Costs
Your neighbour does not pay for the process; you do.
| Situation | Typical cost |
|---|---|
| Neighbour consents in writing | No surveyor needed |
| One agreed surveyor for both sides | £700 to £1,500 per neighbour |
| Each side appoints its own surveyor | about £1,500 to £3,000 per neighbour |
| Schedule of condition on its own | £200 to £600 |
These are national guide figures for a straightforward extension, not quotes.
Making it easier
Talk to the neighbours before the notice arrives. Show them the drawings, explain the crane day and how long the noisy part will last. A modular extension helps here: most of the build happens in a factory, so the disruption next door is a few weeks rather than a few months.
Offer a schedule of condition even if they consent. It records cracks and stains in their rooms before work, which protects you from claims later.
If you are still weighing up whether a factory-built extension suits your house, our overview of modular extensions London owners are choosing covers the access and planning side too.