Underpinning is one of those jobs that tends to come up in three situations: the building is showing signs of foundation movement, someone wants to form a basement, or a significant extension or loft conversion is increasing the load on foundations that weren’t designed for it. In London, all three are common – particularly the first two.

What underpinning involves

Underpinning means deepening or strengthening the existing foundations of a building. The original foundation stays in place – underpinning adds support below it, either by extending it downward or by installing new piles that take the load independently of the original shallow foundation.

The method depends on why it’s needed, what the ground looks like, and what constraints the site has.

Methods we use

Traditional mass concrete underpinning

The original method – sections of ground are excavated beneath the existing foundation in a controlled sequence, and each section is filled with concrete before moving to the next. The sequence matters: you’re working under a load-bearing structure, so sections are done in a specific order to avoid destabilising what’s above.

This is still the right approach for many domestic subsidence cases, particularly where the foundation just needs to go deeper to get below the shrinkable clay. It works well when there’s reasonable access and the depth required isn’t excessive.

Mini pile underpinning

Where traditional underpinning isn’t practical – restricted access, significant depth required, or the site is too tight for excavation equipment – mini piles are installed through or alongside the existing foundation. The piles go down to a competent bearing layer and new ground beams or needles transfer the load.

This is the most common underpinning approach on London terraced houses where access is limited and traditional excavation under a party wall isn’t straightforward.

Beam and base

A reinforced concrete beam is constructed at a lower level, spanning between new concrete bases. The beam picks up the load from the existing wall and transfers it to the bases. Used where the structure needs more substantial support than mass concrete underpinning provides, or where the geometry of the existing foundation makes traditional underpinning difficult.

Underpinning for basements

Basement formation is now one of the most common reasons we carry out underpinning in London. To excavate below the existing ground floor level, the existing foundations need to be deepened first – otherwise you’d be excavating the ground that’s currently holding the building up.

Basement underpinning is technically demanding and the sequencing needs to be right. It almost always requires party wall agreements with neighbours and a structural engineer overseeing the design. We work alongside structural engineers throughout – the underpinning specification comes from them, and we install to that specification.

More on our basement construction page.

Signs that underpinning might be needed

Not all wall cracks mean foundation problems, but some patterns are worth taking seriously:

• diagonal cracks running from corners of windows or doors, wider at the top than the bottom
• cracks that have opened and then closed seasonally over several years
• doors or windows that have started sticking or no longer close properly
• visible lean or bow in an external wall
• cracks that are still active and growing

A structural engineer should assess the building before any underpinning is specified. The cause of movement needs to be understood first – underpinning without addressing the cause is unlikely to be a permanent fix.

Party wall considerations

Most underpinning in London involves working within 3–6 metres of a neighbour’s foundation, which triggers the Party Wall etc. Act 1996. A party wall notice needs to be served before work starts, and if the neighbour dissents, a surveyor is appointed. This adds time to the programme – factor in at least 2 months from serving notice before works can begin in a contested situation.

We’re used to working on projects where party wall obligations are in play and can advise on the practical implications for programme and access.

If you’re dealing with foundation movement or planning a project that involves underpinning, get in touch. Send us the site address and a brief description of what you’re dealing with – or drawings if you have them – and we’ll advise on the right approach.

📞 +44 7821 836407  |  info@khb-piling.co.uk