Building an extension or basement next to a property that already has deep foundations can create an unusual Party Wall situation.
Homeowners sometimes assume that because the neighbour’s foundations are deeper, Section 6 cannot apply.
That may be true in some situations, but the geometry still needs to be checked properly.
Foundation depth is crucial
The three-metre test under Section 6 considers whether your excavation will extend below the level of the neighbouring foundations.
If the neighbour has a basement significantly deeper than your proposed excavation, that particular test may not be satisfied.
But that should be established using reliable information.
Do not guess the neighbour’s basement depth
A visible basement floor level does not always reveal the depth or shape of the foundations below it.
Structural drawings, historic information or reasonable investigation may assist.
Six-metre rule
Very deep excavations may also need to be considered against the separate six-metre test.
Our Section 6 Party Wall guide explains both provisions.
Existing basement walls
Where the neighbouring basement sits directly against the boundary, your works may also interact with:
- retaining walls;
- waterproofing systems;
- drainage;
- external wall structures; or
- foundations extending beneath boundary areas.
These details can have significant practical importance even where the basic Party Wall geometry appears straightforward.
Do not damage waterproofing
Below-ground waterproofing systems can be particularly sensitive.
Construction adjoining an existing basement should therefore consider how excavation, drilling and new structures could affect the neighbour’s waterproof envelope.
Simple Survey can review the relationship
Projects next to existing basements should be assessed using accurate sections rather than assumptions.
Send us the proposed excavation drawings and any available neighbouring information and Simple Survey can advise on the Party Wall position.