The 3 metre and 6 metre rules under Section 6 of the Party Wall etc. Act 1996 decide whether your excavation works require notice to be served on a neighbouring owner. If you are planning a basement, an extension or new foundations close to an adjoining property, understanding these two tests is the first step to staying compliant.
What Section 6 Covers
Section 6 applies to excavation and construction near a neighbour’s building or structure, regardless of whether a party wall is involved. The concern is the ground and the support it provides to your neighbour’s foundations. Two distance-and-depth tests determine when a notice is required. Only one test needs to be met for the duty to serve notice to arise.
The 3 Metre Rule
The 3 metre rule is engaged where you propose to excavate within three metres measured horizontally from any part of a neighbouring owner’s building or structure, and any part of that excavation will go deeper than the bottom of their foundations. Picture a vertical line dropped from the nearest point of your neighbour’s wall: if your dig sits within three metres of that line and reaches below the underside of their footings, notice is required.
The 6 Metre Rule
The 6 metre rule reaches further but is steeper. It applies where you excavate within six metres of a neighbouring building or structure and any part of your excavation would meet a line drawn downwards at 45 degrees from the bottom of their foundations. In practice this catches deeper works, such as basements and piled foundations, where the influence of your dig extends outward and downward toward your neighbour’s support.
Notice Periods at a Glance
| Test | Trigger | Notice period |
|---|---|---|
| 3 metre rule | Excavation within 3m and deeper than neighbour’s foundations | 1 month |
| 6 metre rule | Excavation within 6m meeting a 45° line from foundations | 1 month |
A Section 6 notice must be served at least one month before works begin and should state your intentions, including whether you propose to underpin or otherwise strengthen the neighbour’s foundations. Plans and sections showing the depth and siting of the excavation must accompany the notice. You can read more about serving notice on our party wall notices page.
Why It Matters for Basements and Extensions
Basement projects almost always trigger Section 6 because they involve deep excavation close to neighbouring walls. Rear and side-return extensions frequently do too, particularly where new strip or trench-fill foundations sit near the boundary. Serving the correct notice protects you as the building owner: it establishes a clear record of the neighbouring property’s condition through a schedule of condition and results in a binding party wall award that governs how the work proceeds.
- Confirms the depth of neighbouring foundations before you dig
- Records the existing condition of adjacent structures
- Sets out safeguards such as monitoring or temporary support
- Reduces the risk of disputes over alleged damage
If you are unsure which test applies, it is always safer to have the geometry checked by an RICS-accredited surveyor. Where notice is required and not served, the adjoining owner may seek an injunction halting your works. To understand who bears the cost and how the process unfolds, see our guide to the costs and the process.
FAQs
Do I need a notice if my excavation is shallow?
If your excavation stays above the bottom of your neighbour’s foundations and falls outside the 45 degree line, neither test is met and no Section 6 notice is required. The depth relative to their footings is the deciding factor, not simply how close you dig.
How do I know how deep my neighbour’s foundations are?
Foundation depth is often unknown at the outset. A prudent approach is to assume the tests may be engaged and serve notice, or to carry out trial holes. Your appointed surveyor will advise on the safest course and record findings in the award.
Does the 6 metre rule always mean underpinning?
No. The rule simply requires notice; it does not dictate a particular method. The award will specify any protective measures the engineer and surveyors consider necessary for your specific project.
If you are planning excavation works near a neighbouring property anywhere in London, our RICS-accredited surveyors can confirm whether Section 6 applies and manage the notices and award for you. Contact us to discuss your project.