Extension Party Wall Calculator — Notice Requirements

You need to serve notice if: building on or at the line of junction (boundary) with a neighbour (Section 1), carrying out work on an existing party wall or structure (Section 2), or excavating within 3m or 6m of an adjoining building below their foundation level (Section 6). The Party Wall etc. Act 1996 applies to England and Wales.

The 3m rule: if you excavate within 3m of a neighbouring building and go deeper than their foundations, a Section 6 notice is required. The 6m rule: if you excavate within 6m and the excavation goes below a 45-degree line drawn from the bottom of their foundations, notice is also required. These rules protect neighbours from subsidence.

The building owner (the person doing the work) pays all surveyor fees, including the adjoining owner's surveyor. Fees are not fixed by the Act — under Section 10 they must simply be reasonable, so figures are negotiated case by case. As an indicative 2026 market estimate, a straightforward agreed-surveyor matter is roughly £800–£1,500 per adjoining owner, rising to £3,000+ where each party appoints their own surveyor or for basements and complex work. Agreeing a single 'agreed surveyor' keeps costs lower.

If the adjoining owner does not respond within 14 days, they are deemed to have dissented. This means party wall surveyors must be appointed. Do not proceed without serving notice — ignoring the Party Wall Act can lead to injunctions, compensation claims and difficulties selling the property.

Yes, if your extension is not on the boundary, does not affect an existing party wall, and excavation does not go below neighbouring foundations within 3m/6m. Even when notice is not legally required, it is good practice to inform neighbours of your plans and keep a record of the condition of their property before work starts.