Footings Close to Neighbouring Foundations

When you build an extension close to a boundary, there is often very little horizontal distance between your new foundation and your neighbour’s existing one. Getting this wrong can cause your neighbour’s foundation to lose support and lead to settlement, cracking and a very difficult conversation. Here is how to approach it properly.

The 45-Degree Rule

The key principle is that excavating for a new foundation must not undermine an existing one. The standard engineering rule is that the new foundation formation level should stay above a 45-degree line drawn from the underside of the neighbouring footing.

In practical terms: if your neighbour’s foundation base is, say, 600 mm below ground level, and your new excavation is 600 mm away horizontally from their foundation face, you can safely excavate to a depth of up to approximately 1000 mm below their formation level before you risk destabilising their footing. The further away you are horizontally, the deeper you can safely go.

If you cannot stay above that 45-degree line — because the ground level difference between the two sites demands it, or because the site is very close to the boundary — you need a different approach.

Staged (“Hit and Miss”) Excavation

Where the 45-degree rule cannot be respected without compromising the new foundation’s required depth, the standard solution is staged excavation:

1. Divide the new foundation into bays no wider than 1 metre
2. Excavate and pour alternate bays first (leaving gaps between them)
3. Allow the concrete to cure fully (typically at least 48 hours)
4. Excavate and pour the remaining bays

The key requirement between adjacent pours is a minimum spacing of 2–3 metres — you should not be excavating adjacent bays at the same time. This staged approach ensures there is always a mass of ground between the new work and the neighbour’s foundation, preventing undermining.

Practical Implications for Your Builder

Your builder needs to understand this before starting excavation. The implications are:

– More visits to complete the foundation pour (at least two, sometimes more)
– Concrete delivery must be planned for the size of each bay, not the full length
– The entire excavation cannot be open at once

This adds time and some cost, but it is the correct and safe way to work close to neighbouring structures.

Party Wall Act Considerations

Work close to neighbouring foundations almost always triggers the Party Wall etc. Act 1996. You should take advice on whether a party wall notice is required before work starts. Your structural engineer can note the foundation proximity implications, but party wall matters are handled separately by a party wall surveyor.

Designing Your Close-Boundary Foundation

PorthouseDean designs foundations for extensions close to boundaries, including the specification of appropriate depths and staged excavation requirements. Get a quote to get started.