What Is Underpinning and When Is It Needed?

What Is Underpinning and When Is It Needed?

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Underpinning is one of those construction terms that carries significant weight — financially and structurally. For London homeowners, it comes up most often in the context of subsidence, basement projects, or the discovery that older foundations are simply not deep or strong enough for what the building needs to do.

Here is a clear explanation of what underpinning is, when it is required, and how the process works.

What Underpinning Actually Does

The word means exactly what it suggests: strengthening the foundation under the existing structure. Specifically, underpinning transfers the load of the building from its current foundation — which may be shallow, compromised, or no longer bearing on stable ground — to a deeper, more stable stratum below.

It is not a repair to the wall above. It is an intervention at foundation level, typically starting from below the existing foundation and working downward to reach bearing material capable of supporting the building loads safely.

Why a Building Might Need Underpinning

Subsidence. In London, the most common cause is seasonal movement of London Clay. The Eocene-age London Clay that underlies most of Inner and Greater London is highly susceptible to shrink-swell behaviour: it loses volume during dry summers (particularly pronounced in drought years) and swells again in wetter winters. This cyclical movement causes differential settlement, particularly where mature trees are extracting moisture from the clay within root-strike distance of the foundations. Structural engineers use the RICS crack classification system — Classes 0 through 5 — to assess severity. Category 3 (crack widths over 5mm, distortion beginning to show) and above typically triggers structural engineer involvement and insurers may require remediation.

New basement excavation. When a basement is created adjacent to an existing foundation — particularly a neighbour's or the existing structure's own shallow foundation — the act of excavating removes the lateral support that was keeping that foundation stable. Underpinning transfers the load before excavation takes place, so the existing structure doesn't move as the ground is removed beside it.

Additional load. If a significant additional storey is being added to an existing building, the foundations that were designed for the original load may not be capable of carrying the increased load. A structural engineer will assess bearing capacity and determine whether the existing foundation is adequate or needs strengthening.

Historically shallow foundations. Pre-war residential construction in London — Edwardian, Victorian, and earlier — often used shallow strip foundations sitting at depths that would not meet current standards. If these buildings are being significantly altered or extended, a structural engineer may flag inadequate foundation depth as part of the pre-construction assessment.

Methods of Underpinning

Mass concrete (sequential pit method). The traditional and most common method for residential underpinning in the UK. The existing foundation is excavated in bays — typically alternating bays across the length being underpinned, so no more than one-third of the foundation is unsupported at any one time. Each bay is excavated to the required depth, filled with mass concrete, and allowed to cure before the adjacent bays are started. The sequential approach ensures the building remains supported throughout. This method is well-suited to older strip foundations and relatively modest depths.

Mini-pile underpinning. Where the required bearing stratum is very deep (12 metres or more is not unusual in parts of London), or where access to excavate in bays is restricted, mini-piles are drilled through and below the existing foundation. Steel or concrete piles are installed and capped, transferring the building load downward. This is the method typically used for basement underpinning in London where the water table and deep bearing stratum require it.

Resin injection. A less invasive method where expanding polyurethane resin is injected at depth through small-diameter drill holes. The resin compacts loose soils and can raise and re-level a structure that has undergone minor settlement. Suitable for certain ground conditions and minor movement — not appropriate for severe structural failure or where significant additional depth is required.

Party Wall Considerations

Underpinning that encroaches within three metres of a neighbouring building's foundation triggers the Party Wall etc. Act 1996 section 6 (the "three to six metre" notice provisions). This is separate from the section 2 Party Wall Notice that applies to work on the party wall itself. A Party Wall Surveyor must be appointed, and a Party Wall Award agreed, before this work commences. Failure to serve the correct notice can result in an injunction stopping the works.

Engineering and Building Regulations

Underpinning design is an engineering decision — not a builder's design-and-build item. A Structural Engineer (chartered with IStructE or ICE) must design the underpinning scheme, specifying depths, bay widths, concrete mixes, and reinforcement where required. The Building Regulations Part A (structure) governs; a full plans application to LABC or an RBCA is required, and the inspector must attend at foundation formation level — before backfill — to confirm the bearing stratum and formation depth.

How RCB Coordinates Structural and Ground Works

Where a survey flags potential movement, where a basement project is on the horizon, or where a structural engineer's report recommends foundation investigation, RCB engages the structural engineer early in pre-construction — before the main contract is placed. The underpinning scheme, Party Wall notice programme, and Building Regulations application are managed as part of the pre-construction package rather than discovered mid-build. Ground conditions and structural requirements are not the kind of surprise you want after the programme has started.

If your property has shown signs of movement, or you are planning a basement or significant structural project, speak with RCB at the earliest stage to ensure the foundation picture is clear before work begins.