Glossary
Underpinning London foundations for subsidence or a basement
Strengthening or deepening existing foundations. Required for many basement extensions.
Strengthening or deepening existing foundations. Required for many basement extensions.
Underpinning extends or deepens an existing foundation to transfer load to more competent ground. It is used to remedy subsidence, to take a foundation below a neighbouring excavation, and — most commonly in London — to gain head height when lowering a basement floor.
Traditional mass concrete underpinning in a hit-and-miss sequence of bays, typically 1 m to 1.2 m wide, no more than a quarter of the wall length unsupported at any time, is the domestic norm. Beam-and-base and mini-piled underpinning are used where ground conditions or access rule out mass concrete.
The sequence is what keeps the wall up. Excavating adjacent bays simultaneously is the classic and dangerous error, which is why the method statement is part of the structural package and not a site decision.
Underpinning a party wall is section 2 work under the Party Wall etc. Act 1996, requiring one month's notice. Excavating within 3 m of a neighbouring structure and below its foundations is section 6 work requiring two months' notice. A basement usually triggers both, plus a schedule of condition and often a ground-movement assessment.
Underpinning is priced per bay or per linear metre and is one of the most expensive elements of a basement project. It is also the slowest: the hit-and-miss sequence, curing times and pinning-up between the new and old concrete mean the programme is measured in weeks per elevation, not days.
How underpinning plays out in Newham: London Borough of Newham (Newham Council) maintains 10 conservation areas, the Manor Park Conservation Area and the Forest Gate Conservation Area among them, and an Article 4 direction is in force there, so permitted development cannot be assumed. Anyone dealing with underpinning at a Newham address is therefore working to a £548 householder application decided in 8 weeks, or a £249 prior approval over 6 weeks where underpinning still qualifies.
| Common method | Mass concrete, hit-and-miss bays |
|---|---|
| Typical bay width | 1–1.2 m |
| Maximum unsupported | One quarter of the wall length |
| Party wall notice, section 2 | One month |
| Excavation notice, section 6 | Two months |
| Alternatives | Beam and base, mini-piled underpinning |
Almost always, unless the existing foundations are already deep enough.
Yes — underpinning a party wall is section 2 work.
Correctly sequenced it should not, which is why the method statement and schedule of condition matter.
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