Do You Need a Party Wall Notice?

Whether you need a party wall agreement comes down to three questions: does your extension modify a wall you share with a neighbour, does it build on the boundary line, and do your foundations dig within 3 metres of a neighbour's building. If any answer is yes, the Party Wall Act 1996 applies and you must serve a formal party wall notice before any work starts. This checker walks you through those triggers and tells you which sections of the Act are in play.

The surprise for most homeowners is the third trigger. You can build a rear extension that never touches the shared wall and still need notice, purely because of how deep and how close your foundations sit to next door.

The three triggers

The Act splits into sections, and an extension can set off more than one at a time.

Section 1 covers building a new wall on or right at the boundary line, common with a side-return extension. It carries a one month notice period. Section 2 covers work on an existing shared wall: cutting in steelwork, removing a chimney breast, or raising the wall. That work needs two months notice. Section 6 covers excavation, and it is the one that catches nearly everyone. If you dig within 3 metres of a neighbour's structure and your foundations go deeper than theirs, or within 6 metres where a 45-degree line drawn down from their footings would cross your dig, you must serve notice. Older houses often sit on footings only 0.6 to 0.9m deep, while a modern trench foundation runs 1 to 1.2m, so the excavation rule bites on most extensions. These are the Section 6 distance rules, and your structural engineer can confirm the exact geometry once foundation depths are fixed.

What it costs

The good news is that the cheapest route is by far the most common. If your neighbour signs a written consent, the notice costs nothing and your only sensible spend is a schedule of condition survey, a photographic record of their property taken before you break ground. Budget £0 – £600 per neighbour for that.

Costs climb only if a neighbour formally dissents, or fails to reply within 14 days, which the Act treats as automatic dissent. At that point a party wall surveyor has to be appointed to prepare a Party Wall Award, and you pay the bill. Pushing for a single agreed surveyor acting fairly for both sides keeps this to around £700 – £1,500 per neighbour. If each side insists on their own surveyor, expect £1,500 – £4,000 per neighbour, and more again if the two cannot agree and a third surveyor steps in to arbitrate.

A worked example

Take a semi-detached house planning a single-storey rear extension. The extension does not cut into the shared wall, so Section 2 stays clear. But the foundations run about 1m deep and sit within 3 metres of next door, well below the neighbour's shallow original footings. That trips Section 6, so a notice is required.

The homeowner talks to the neighbour first, shows them the drawings, and the neighbour agrees. They sign a written consent and a surveyor takes a schedule of condition of the neighbour's kitchen wall. Total party wall spend lands at the £0 – £600 consent figure, and the build starts on time.

Had the neighbour dissented instead, the same job would have moved onto the agreed-surveyor path at £700 – £1,500, or the two-surveyor path at £1,500 – £4,000 in the worst case, and added roughly two extra months to the timeline. That gap is exactly why the party wall process should start the week your architect delivers drawings, running in parallel with your building control application rather than after it.