What Is Underpinning?
Underpinning is the process of strengthening or deepening the foundations of an existing building. It is carried out when:
- The existing foundations are no longer adequate to support the loads on them
- The ground below the foundation has moved, shrunk, or been disturbed
- A basement or lower level is being excavated, requiring deeper support
- An adjacent excavation (neighbour's basement, road works) has undermined the existing foundation
Why London Homes Are Susceptible
London has a specific geological challenge: a large proportion of the capital is underlain by London Clay, a highly plastic clay soil that expands when wet and shrinks when dry. This seasonal movement — shrink-swell — can cause significant differential settlement of shallow foundations.
Common causes of foundation movement in London:
- Tree roots: Large trees (especially poplars, willows, and oaks) draw water from the clay, causing shrinkage under and near the foundation. This is the single most common cause of subsidence claims in London.
- Leaking drains: Water from a broken drain softens the ground below the foundation, causing settlement
- Made ground: Some parts of London are built on filled ground (bomb craters, historic quarries, former marshland) that compresses over time
- Adjacent works: Neighbour excavations or basement construction can affect the ground condition adjacent to your foundations
Methods of Underpinning
Mass Concrete Underpinning (Traditional Method)
The traditional approach involves excavating the ground below the existing foundation in sequence — typically one-third of the building width at a time — and filling each section with mass concrete before moving to the next. This extends the foundation to a greater depth where the ground is more stable.
Advantages: Proven method, no specialist plant required, can be closely supervised
Disadvantages: Slow, requires working in confined spaces, involves significant hand-digging
Beam and Base Underpinning
A reinforced concrete beam is cast at foundation level, transferring loads to a series of mass concrete bases or piers at intervals. Less common but useful where selective loading points are feasible.
Mini-Pile Underpinning
Steel or concrete mini-piles are installed through the existing foundation into stable ground (typically at 8–15 metres depth below London Clay). The foundation is then connected to the pile tops with a reinforced concrete beam.
Advantages: Less excavation, faster, can reach stable ground well below the clay shrink-swell zone
Disadvantages: Requires specialist plant, more expensive per metre of support
Resin Injection
A two-component expanding resin is injected into the ground below the foundation via tubes, compacting loose soil and stabilising the ground. Used for minor subsidence in less severe cases.
Advantages: Fast, minimal disruption, no excavation
Disadvantages: Not appropriate for significant structural movement or major foundation failure
Subsidence and Insurance
If your property shows signs of subsidence — stepped diagonal cracks, doors or windows sticking, visible movement in floors — the first step is typically an insurance claim rather than immediate underpinning.
Your buildings insurer will:
- Appoint a loss adjuster to assess the damage
- Commission a structural engineer's report
- Monitor the movement (typically for 12–18 months) to establish whether it is ongoing or historic
- Authorise underpinning only if monitoring confirms ongoing movement
Important: Do not carry out any repair work (including underpinning) without involving your insurer first. Unauthorised repairs can invalidate your claim.
When Underpinning Is Needed Without Subsidence
For basement conversion projects, underpinning is typically required to:
- Lower the floor level (the existing foundation is at shallow depth, and the new floor will be below it)
- Carry increased loads if the basement will be extended in footprint
In this context, underpinning is planned structural work rather than a distress repair, and is engineered accordingly.
Typical Costs
Underpinning costs vary enormously depending on the method, depth, site access, and extent of the area to be treated.
| Method | Indicative Cost Range |
|---|---|
| Mass concrete (per linear metre of wall) | £500–£1,500/m |
| Mini-pile underpinning | £1,500–£4,000 per pile |
| Resin injection | £3,000–£10,000 per property |
| Basement underpinning (full scope) | £20,000–£80,000+ |
These are rough indicative figures only — a structural engineer's specification and competitive contractor quotes are essential for any real project.
RCB Design & Build
RCB works with specialist structural engineers and underpinning contractors for projects where foundation works are required. We manage the pre-construction assessment, structural specification, Building Control application, and site supervision to ensure this complex work is carried out safely and to programme.