The Party Wall Act 3 Metre Rule

How the rule works

The rule combines a distance test and a depth test, and both must be satisfied for the notice requirement to bite.

When the 3 metre rule applies

The following are typical examples:

• Any excavation within three metres of an adjoining owner's building or structure that goes deeper than the bottom of their existing foundations.
• Works for basement construction, lift pits, lightwells and underpinning along a neighbouring boundary.
• Foundations for rear or side extensions where the proposed trenches are deeper than the neighbour's.
• New retaining walls and drainage runs that cut below adjoining foundations within the three metre zone.
• Demolition works where subsequent excavation will fall within the statutory geometry.

If the proposals include deep piles or a basement, the section 6 guide should be read alongside this one, because the 6 metre rule frequently applies as well.

What the notice must contain

A section 6 notice — required whether the 3 metre or 6 metre trigger applies — is a formal statutory document. Defects in content or service can invalidate it and force the notice to be re-served.

• The name and address of the building owner.
• The address of the site on which the works are proposed.
• A description of the proposed excavation, with plans and sections.
• The proposed depth of the excavation and its position in relation to the neighbouring building.
• The proposed start date, giving at least the statutory one month's notice.

For the general rules on party wall notices, see the party wall notice guide.

What happens after service

The adjoining owner has 14 days from service to respond. They can consent in writing, dissent in writing, or do nothing — which the Act treats as a deemed dispute. In a dispute, surveyors are appointed and a party wall award is drawn up before excavation can start.

The Award typically records the existing condition of the neighbour's property, sets the manner and timing of the excavation, and provides a mechanism for resolving any damage that might be caused.

How it differs from the 6 metre rule

The 3 metre rule is the most commonly quoted, but section 6 has a second limb. The 6 metre rule applies where excavation within six metres of an adjoining building would cut below a line drawn down at 45 degrees from the base of their foundations. It typically catches deeper projects such as piled basements and lift pits.

Both rules can apply to the same project. Where they do, the notice still only needs to be served once, but the plans must demonstrate the relationship between the proposed works and both geometric tests.

Common mistakes

• Assuming the rule only applies to basements. Shallow trenches for extension foundations can be caught just as easily.
• Measuring the three metres from the wrong point — it is measured from the nearest part of the neighbour's building, not the boundary.
• Treating the neighbour's ground level as a proxy for their foundation depth. The correct reference point is the bottom of their foundations.
• Skipping the notice because the neighbour has been told verbally. Verbal warnings are not statutory service.
• Starting excavation during the 14 day response period, which turns a procedural issue into an injunction risk.