Basement construction and deep excavation are among the most technically demanding works undertaken in London, and they almost always engage Section 6 of the Party Wall etc. Act 1996. As RICS-accredited party wall surveyors covering the whole capital, we help owners manage the heightened risk to neighbouring properties and secure a fair, protective award.
Why basements are high-risk works
Digging below the level of a neighbour’s foundations can remove lateral support from the ground and, if poorly managed, cause settlement, cracking or worse. London’s mix of clay soils, high water tables and Victorian foundations makes basement projects particularly sensitive. This is precisely the risk the Act’s excavation provisions are designed to control, and why a thorough surveyor-led process matters.
Common basement and excavation works
- Excavating to form a new basement or to lower an existing cellar.
- Underpinning the party wall to take up new loads or deeper foundations.
- Installing piled or contiguous retaining walls close to the boundary.
- Forming lightwells and sub-basement extensions under gardens.
Section 6 and the notice you need
Section 6 applies when you excavate within three metres of a neighbouring structure and to a lower depth than their foundations, or within six metres if a line drawn down at 45 degrees from the base of their foundation would strike your excavation. Our 3 and 6 metre rule guide explains this in detail. Where it applies, you must serve a Section 6 notice at least one month before excavation begins, using the correct form of party wall notice. The notice must state the depth of the proposed excavation and whether you intend to underpin or strengthen the neighbour’s foundations, accompanied by plans and sections.
| Distance / depth trigger | Applies? | Notice period |
|---|---|---|
| Within 3m and deeper than neighbour’s foundations | Yes — Section 6(1) | 1 month |
| Within 6m and below a 45° line from their foundation | Yes — Section 6(2) | 1 month |
| Works also cutting into the party wall | Section 2 as well | 2 months |
Security for expenses
For high-risk excavation, an adjoining owner is entitled under Section 12 to request ‘security for expenses’ — a sum set aside, often in an escrow or bond, to ensure that if the building owner runs out of funds mid-dig, the neighbour’s property can still be made safe. Determining a reasonable figure is a matter for the appointed surveyors, and it is a routine feature of well-run party wall awards for basement schemes.
Monitoring and the schedule of condition
Because ground movement is the principal risk, basement awards frequently require monitoring throughout the works. This may include:
- A detailed photographic schedule of condition of the neighbour’s property before any dig.
- Movement monitoring points, crack gauges or precise levelling stations.
- Trigger levels with agreed actions if movement exceeds set thresholds.
- Method statements and temporary works designs reviewed as part of the award.
These measures protect both building owners and adjoining owners, giving early warning long before cosmetic damage becomes structural.
Two surveyors and the third surveyor
Given the stakes, most basement disputes are resolved with each party appointing its own surveyor rather than sharing an agreed surveyor. A third surveyor is selected at the outset to determine any matter the two cannot agree. Because the excavation is for the building owner’s benefit, they normally meet the reasonable fees; our costs and process guide sets out what to budget.
FAQs
Can my neighbour stop my basement?
No. The Act does not give a neighbour a veto; it gives a framework to regulate the works and protect their property. A dissent simply triggers the appointment of surveyors and an award.
What is security for expenses in practice?
It is a reasonable sum, determined by the surveyors, held so the neighbour is not left exposed if funds run out. It is released once the relevant works are safely completed.
How long does a basement award take?
Allow several weeks from notice to award, as monitoring regimes, method statements and security terms all need careful agreement. Serving notice early avoids delaying your start on site.
Discuss your basement project
Basement and excavation works reward careful, experienced party wall management. Our RICS-accredited surveyors will serve the right notices, agree robust monitoring and protect all parties. Please contact us to talk through your London basement scheme.