The short answer
Underpinning strengthens or deepens existing foundations that are no longer adequate — usually because of subsidence, a deeper extension alongside, or ground movement — whereas new foundations are built fresh for new structure. You need underpinning when the existing building is sound but its footings have failed or must be matched to a deeper neighbour; you need new foundations when you are building something new. The right answer depends entirely on a structural engineer's diagnosis of the cause: subsidence from a leaking drain, clay shrinkage near a tree, or made-up ground each call for different remedies, and treating the symptom without the cause wastes money. Underpinning is disruptive and costs from roughly £1,000–£2,000+ per linear metre, so it is typically a few thousand to well over £15,000 for a section of wall. The work is structural and must satisfy Part A with Building Control involved.
Underpinning and new foundations solve different problems. Confusing the two is expensive, so the first step is always to find out why the ground or the existing footing has failed.
Underpinning at a glance
- What it doesDeepens / strengthens existing footing
- Common triggerSubsidence, deeper neighbour, movement
- Typical cost~£1,000–£2,000+ per metre
- First stepEngineer diagnoses the cause
- Sign-offBuilding Control + Part A
Underpinning vs new foundations
- Underpinning extends an existing foundation deeper or wider — often by excavating and casting concrete beneath it in short, sequenced bays — to reach firmer ground or carry more load.
- New foundations are built from scratch for a new wall, extension or building, designed for that structure and ground.
You may need both on one project: for example, new trench fill for an extension, plus underpinning a short length of the existing house wall so the two sit at a compatible depth. An engineer specifies where each applies.
What it costs and what drives it
Underpinning is priced by the metre and by method. Traditional mass-concrete underpinning in sequenced bays is the most common domestic approach; beam-and-base or piled underpinning is used for heavier loads or deeper failures and costs more. Access, depth, the number of bays and whether you are dealing with an active subsidence claim all move the figure.
| Item | Indicative figure | Notes |
|---|---|---|
| Mass-concrete underpinning | ~£1,000–£2,000+/m | Most common domestic method |
| Section of wall (typical) | ~£5,000–£15,000+ | Depends on length & depth |
| Piled / beam underpinning | Higher | Heavier loads or deep failure |
| Engineer + investigation | Separate fee | Always needed first |
Indicative UK ranges for guidance only. Sources: Checkatrade underpinning cost guide; LABC subsidence guidance.
Why the cause must come first
Underpinning treats the foundation, but cracking usually has a root cause: a leaking drain washing out soil, clay shrinkage from a nearby tree, or settlement of made-up ground. Underpin without fixing a leaking drain and the problem returns. A structural engineer should investigate — including drain surveys and trial holes — before any remedy is chosen, and that diagnosis decides whether you need underpinning, drain repairs, tree management, or in some cases nothing structural at all. Because this is structural work affecting an existing building, Building Control must be involved and the design must meet Part A.
Alternatives to underpinning, and when you genuinely need new foundations
Underpinning is not the only structural remedy, and it is often the most disruptive, so a good engineer considers the alternatives first.
- Fixing the cause alone: repairing a leaking drain or managing a thirsty tree can sometimes stabilise minor movement without underpinning, especially if monitoring shows the movement has stopped.
- Resin (geopolymer) injection: on suitable ground, an expanding resin injected beneath the footing can re-compact soil and lift settlement, sometimes faster and less disruptively than traditional underpinning — though it is not right for every cause or soil.
- Crack monitoring: tell-tales or survey points tracked over months establish whether movement is active or historic; historic, stable cracking may need only cosmetic repair.
You move to new foundations rather than underpinning when you are adding new structure — an extension, a new wall, a load-bearing change — that needs its own footing designed for its loads and ground. The two frequently appear on the same project: a new trench fill or piled foundation for the extension, plus a short length of underpinning where it meets the existing house so the two are at compatible depths. The deciding document is always the structural engineer's report. Treating cracking as automatically meaning "underpin the whole house" wastes money; treating new structure as something that can sit on old, inadequate footings risks failure. Get the diagnosis, then match the remedy — underpinning, an alternative, new foundations, or a combination — to what the ground and the building actually need.
How traditional underpinning is actually carried out
It is worth knowing how mass-concrete underpinning is done, because the method explains both the cost and the disruption. The existing footing is deepened in short, sequenced sections — usually called the hit-and-miss or bay sequence — so the wall is never undermined all at once.
- Sequenced bays: the engineer divides the wall into numbered bays (often around a metre wide) and specifies an order so that no two adjacent bays are open at the same time. One bay is excavated below the existing footing, cast in concrete, allowed to gain strength, then the next non-adjacent bay is done, and so on.
- Pinning up: a gap is left between the top of the new concrete and the underside of the old footing, then "pinned" with a dry, rammed mortar (or a non-shrink grout) so the load transfers onto the new, deeper base.
- Working space and spoil: the work is done by hand in confined pits, which is slow and labour-intensive, and the excavated soil has to be removed — both reasons underpinning is priced by the metre and is more expensive than it looks for a short length.
Because the work is structural and affects an existing building, it must be engineer-designed and supervised, notified to Building Control under Part A, and inspected as it proceeds. Where the wall is on or near a boundary, the Party Wall etc. Act 1996 may also require notice to a neighbour. None of this is a reason to avoid underpinning where it is genuinely needed — but it does explain why the right first step is always an engineer's diagnosis, so you only underpin what actually has to be underpinned, in the way the building and the ground require.
Frequently asked questions
How do I know if I need underpinning?
You do not decide it yourself. A structural engineer investigates the cracking — its pattern, drains and ground — and only recommends underpinning if the foundation itself is inadequate. Many cracks are not structural and need no underpinning at all.
Is underpinning covered by insurance?
If the damage is caused by subsidence, buildings insurance often covers investigation and repair, subject to your excess. Insurers usually appoint their own engineer, so report it before starting any work.
Does underpinning affect future house sales?
A history of underpinning must be disclosed and can affect insurance and buyer confidence, but properly engineered, certified and signed-off work is a recognised, insurable repair. Keep the engineer's report and Building Control documents.
Sources & further reading
- Checkatrade — underpinning cost guide
- LABC — subsidence and underpinning
- Planning Portal — Approved Document A (structure)
Figures on this page are typical UK ranges drawn from published sources and depend on your specific site. They are guidance, not a quotation.