Of all the building projects we advise on at ALD Surveying, basement conversions and deep excavations consistently generate the most questions and, when handled incorrectly, the most disputes. The combination of significant structural intervention, proximity to neighbouring foundations, and the relatively specialist nature of the works makes excavation cases some of the most technically demanding within the party wall framework.
When Does Excavation Trigger the Act?
Section 6 of the Party Wall etc. Act 1996 sets out the rules around excavation. Two thresholds are relevant. The first: if you plan to excavate within three metres of an adjoining building and to a depth lower than the bottom of that building’s foundations. The second: if you plan to excavate within six metres of an adjoining building and your excavation cuts a line drawn at 45 degrees downward from the base of their foundations. Either scenario requires you to serve an Adjacent Excavation Notice at least one month before works begin.
In practice, the three-metre rule catches most standard extension foundations built close to a party fence. The six-metre rule is more commonly triggered by basement work, where foundations go deep enough to interfere with that 45-degree line even at greater horizontal distances.
Why Excavation Cases Are More Complex
Soil movement is unpredictable in ways that above-ground building work is not. Even with excellent engineering and careful contractors, excavating near a neighbouring building creates a real risk of ground movement, which in turn can cause cracking, settlement, or in extreme cases structural damage to the adjoining property. This isn’t scaremongering. It’s why the Act applies specifically to excavation, and why the Schedule of Condition before works begin is so important in these cases.
At ALD Surveying, we carry out detailed pre-works Schedules of Condition for all excavation projects, documenting every crack, mark, and structural feature of the adjoining property with photography. If a neighbour later claims that the excavation caused damage, this record either confirms or contradicts the claim.
Underpinning and the Party Wall Process
Underpinning, which involves strengthening or deepening existing foundations, is among the most intrusive forms of notifiable work. It is common in basement conversion projects and frequently required when extending a property where existing foundations are insufficiently deep. Underpinning of a party wall or structure requires explicit consent within the Party Wall Award, and the method of works needs to be carefully described.
In our experience, adjoining owners are understandably anxious when underpinning is proposed. Clear communication from both the party wall surveyor and the building owner’s structural engineer is essential to giving neighbours the confidence that the works are being properly managed.
What an Award for Excavation Works Covers
A Party Wall Award for excavation works is typically more detailed than one produced for above-ground party wall works. Beyond the standard description of permitted works and hours, it will usually include requirements around monitoring, specifying at what intervals the adjoining property must be checked for movement and by whom. It may also include provisions for temporary support structures, drainage protection, and requirements that a structural engineer be present during certain phases of the dig.
Getting this level of detail right is not bureaucracy for its own sake. It is the framework that keeps a complex project running without interruption or dispute.
Common Misconceptions
One misconception we frequently encounter is that the Party Wall Act only applies to shared walls, and that if your excavation is entirely within your own curtilage, it falls outside the Act’s scope. This is incorrect. Section 6 applies based on proximity to the adjoining building, not on whether a shared wall is affected. The location of your trench relative to your neighbour’s foundations is what matters, not the boundary line.
Another is that planning permission for a basement conversion implies party wall compliance. It doesn’t. Planning and party wall are entirely separate processes. Permission from the council does not amount to a party wall agreement and vice versa.
For more information on ALD Surveying’s approach to party wall matters involving excavation and basement works, visit our Party Wall Act service page.