Underpinning strengthens or deepens existing foundations, and where it affects a party wall it is governed by the Party Wall etc. Act 1996. As RICS-accredited party wall surveyors serving every London borough, we manage the notices, schedule of condition and award so that underpinning proceeds safely and the neighbour’s property is fully protected.
When is underpinning needed?
Underpinning is used to remedy or pre-empt problems and to enable deeper construction. Common reasons in London include:
- Subsidence caused by clay shrinkage, tree roots or historic leaks.
- Strengthening shallow Victorian foundations before adding storeys.
- Deepening foundations to form a basement or lower a ground floor.
- Correcting differential settlement and stabilising cracked walls.
Where the wall being underpinned is shared, the works are notifiable, and serving the right party wall notice is essential.
Which section of the Act applies
Underpinning a party wall to take up new loads — for example before a loft or additional storey — is a Section 2 activity, notified by a party structure notice served at least two months in advance. Where underpinning forms part of an excavation below the neighbour’s foundations, Section 6 also applies, requiring one month’s notice; see our 3 and 6 metre rule guide. Many schemes engage both, so the notice should describe the whole works.
| Underpinning scenario | Section | Notice period |
|---|---|---|
| Strengthening the party wall for new loads | Section 2 | 2 months |
| Deepening below neighbour’s foundations (excavation) | Section 6 | 1 month |
| Subsidence remediation to a shared wall | Section 2 (and 6 if excavating) | 1–2 months |
Methods of underpinning
The appropriate method depends on ground conditions, loads and access. The award will typically reference an engineer’s design and method statement covering:
- Traditional mass concrete — excavating and filling in short, sequenced ‘hit-and-miss’ bays to maintain support.
- Beam and base — a reinforced concrete beam spreading load onto new bases or piles.
- Mini-piling — small-diameter piles where deeper competent strata are needed, common on constrained London sites.
Sequencing is critical: removing too much support at once risks movement, which is precisely why surveyor oversight and monitoring matter.
The schedule of condition and monitoring
Before any excavation, we prepare a detailed photographic schedule of condition of the adjoining property. For higher-risk works, the party wall award may also require movement monitoring, crack gauges and agreed trigger levels. These safeguards give an impartial baseline and early warning, protecting both building owners and adjoining owners. Because underpinning is a form of foundation excavation, an adjoining owner may also seek security for expenses under Section 12. You can review related works on our types of party wall works page.
Surveyors and fees
If the neighbour dissents or does not reply within 14 days of the notice, a dispute is deemed to arise and surveyors are appointed — an agreed surveyor or one each, with a third surveyor to resolve any impasse. As the works benefit the building owner, they normally meet the reasonable fees; our costs and process guide explains the detail.
FAQs
Is remedial underpinning for subsidence still notifiable?
If it affects a shared party wall, yes. Even repair works that cut into or strengthen the party structure are Section 2 activities requiring notice.
How disruptive is underpinning to my neighbour?
It is sequenced to maintain support at all times, and a schedule of condition plus monitoring keeps risk low. The award sets working hours and access to minimise disruption.
Can my neighbour ask me to underpin their side too?
The Act allows for strengthening a neighbour’s foundations where necessary for the works. Any such measures, and who pays, are determined by the surveyors in the award.
Talk to a party wall surveyor
If your project involves underpinning a party wall anywhere in London, our RICS-accredited surveyors will handle the notices, record a robust schedule of condition and agree a protective award. Please contact us to discuss your requirements.