Underpinning is the process of strengthening or deepening the foundations of an existing building to restore or improve their load-bearing capacity. It is most commonly carried out when a building has suffered subsidence — where ground movement has caused the foundations to settle, leading to cracking and structural distress. Crown Architecture & Structural Engineering Ltd provides structural engineering assessments and design for underpinning projects across London and the South East. This guide explains what underpinning is, when it is needed, and what it costs in 2025.

What Causes Subsidence?

Subsidence occurs when the ground beneath a building’s foundations moves downward, causing the structure above to settle unevenly. Common causes include:

Clay Shrinkage

London and much of southern England overlie shrinkable clay soils. During dry summers, clay dries out and shrinks; it swells back during wet winters. This seasonal movement can cause foundations that sit in or on shallow clay to move with the ground, leading to cracking. Large trees with extensive root systems significantly worsen clay shrinkage by drawing moisture from the soil, sometimes extending shrinkage zones 10–15 metres from the trunk.

Tree Root Activity

The roots of large trees (oaks, willows, poplars) draw significant moisture from clay soils, causing desiccation and shrinkage. If a tree is removed, the soil may rehydrate and swell (heave), which can be equally damaging. Managing tree proximity and root zones is critical in clay-rich areas.

Leaking Drains and Pipes

Underground leaks from drains or water mains can wash away fine particles from granular soils (a process called “piping” or “scouring”), creating voids beneath foundations. This is a common cause of subsidence in urban areas with aging Victorian drainage.

Mining and Tunnelling

Properties in former coal-mining areas may be affected by ground movement as old mine workings collapse over time. Tunnelling for railways or roads can also cause surface settlement.

Inadequate Original Foundations

Many Victorian and Edwardian properties were built with very shallow foundations (as little as 300mm deep) that would not meet modern standards. These may perform adequately in normal conditions but can fail if ground conditions change.

Proximity to Excavations

Excavation for basement conversions, road works, or new development adjacent to an existing building can undermine foundations by removing lateral ground support.

How Subsidence is Diagnosed

A structural engineer or specialist subsidence surveyor will carry out a monitoring programme before recommending underpinning. This typically involves:

• Visual inspection of cracking patterns, widths, and distribution
• Trial hole excavation to expose foundations and assess ground conditions
• Settlement monitoring over 6–12 months using level pins or targets
• Ground investigation (borehole or trial pit) to characterise soil type and moisture content
• Tree survey to assess root proximity and species if clay shrinkage is suspected

Most building insurers require a monitoring period before approving underpinning — active movement must be confirmed before remediation is undertaken.

Methods of Underpinning

Mass Concrete Underpinning (Traditional Method)

The most widely used method for residential properties. The existing foundation is exposed in short sections (typically 1–1.5m wide “pins”), the ground is excavated beneath, and new concrete is poured in to deepen the foundation to a more stable stratum. The process is repeated until all affected sections are underpinned. The pins are then grouted to the existing foundation. This method is labour-intensive, disruptive, and works best where a stable founding stratum is within 2–3 metres of the existing foundation level.

Cost: £1,000–£1,800 per running metre of wall.

Beam and Base Underpinning

A concrete beam is cast beneath the existing foundation and transfers the load onto a series of reinforced concrete bases placed at intervals. This method is used where the founding stratum is deeper or where mass concrete would require excessively large volumes.

Cost: £1,200–£2,000 per running metre.

Mini-Pile Underpinning

Reinforced concrete piles of small diameter (150–300mm) are drilled through the existing foundation into sound ground below. A reinforced concrete needle beam or cap is then cast to transfer loads from the existing wall onto the piles. Mini-piles can reach depths of 6–15 metres, making them suitable for sites with deep deposits of weak or variable ground. They produce less spoil and vibration than large-diameter piles, making them suitable for working in confined spaces.

Cost: £2,000–£4,000 per pile including installation, with typically 4–10 piles per project.

Resin Injection

An expanding polyurethane resin is injected into the ground via small-diameter tubes, filling voids, densifying loose soils, and lifting settled structures. This is a less invasive method that can be used without excavation. It is most effective in granular soils and is increasingly used for minor settlement in lighter structures. Not appropriate for significant structural movement or clay soils.

Cost: £2,000–£6,000 per project depending on extent of treatment.

Screw Pile Underpinning

Helical screw piles are rotated into the ground by a hydraulic torque motor, providing both end-bearing and skin friction capacity. Connection brackets are then attached to the existing foundation, and jacks are used to lift and stabilise the structure. Used extensively in North America and increasingly in the UK for residential subsidence remediation.

Insurance and Underpinning

Most subsidence underpinning projects are funded by buildings insurance. The process involves:

1. Making a claim with your insurer
2. Insurer appoints a loss adjuster and structural engineer
3. Monitoring period (typically 6–12 months)
4. Decision on remediation method by the insurer’s engineer
5. Tendering and appointment of specialist contractor
6. Remediation works
7. Making good of all internal and external finishes

Properties that have been underpinned carry a persistent stigma in the mortgage and insurance market. Some lenders and insurers are cautious about underpinned properties; others accept them subject to evidence that remediation was competently carried out and movement has ceased. Keep all documentation from the underpinning project carefully.

Building Regulations

Underpinning works are structural alterations that require Building Regulations approval under Part A. A structural engineer’s calculations and method statement must be submitted. Building control inspects the works at key stages, including foundation exposure, concrete pours, and grouting.

How Crown Can Help

Crown Architecture & Structural Engineering Ltd provides independent structural engineering assessments for subsidence investigations, underpinning design, and building regulations submissions. We work on behalf of homeowners and insurers and provide expert advice on remediation methods. Call us on 07443804841 to discuss a suspected subsidence problem.

Frequently Asked Questions

How do I know if I have subsidence rather than normal settlement?

Normal settlement produces minor, stable, hairline cracks (typically less than 1–2mm wide) that do not worsen over time. Subsidence cracks are typically wider than 3mm, may be tapered (wider at the top or bottom), often run diagonally from window or door corners, and are actively growing. A structural engineer can assess the pattern and nature of cracking to advise on likely cause.

Does underpinning devalue a property?

A property that has been underpinned with documented, professional remediation and monitored cessation of movement should not be devalued — the problem has been solved. The key is documentation. Properties where underpinning was carried out without records, or where movement is ongoing, will be difficult to sell and insure.

How long does underpinning take?

A typical residential underpinning project using mass concrete method takes 2–6 weeks on site, depending on the extent of the affected area. Including the preceding monitoring period and design process, total project duration from first signs of movement to completion of remediation is typically 12–24 months.

Can I prevent subsidence before it starts?

In clay soils, managing tree proximity is the most effective prevention. Avoid planting large trees close to the house and remove declining trees promptly (but carefully — rapid removal can cause heave). Maintain drainage to prevent leaks from undermining foundations. In mining areas, ground investigation before purchasing or developing can identify risks.

What is heave and how does it differ from subsidence?

Heave is upward ground movement — the opposite of subsidence. It typically occurs when clay soils rehydrate after a tree is removed (the soil swells as moisture content rises), or when trees close to a building are felled. Heave can damage foundations as severely as subsidence. It cannot be remediated by underpinning; the structure must be allowed to reach a new equilibrium or designed to accommodate the movement.