When underpinning is needed
Three main scenarios drive underpinning in London. (1) Subsidence repair — a wall has cracked or settled because of foundation movement, typically caused by clay shrinkage in dry summers, tree-root activity (London plane and oak are common culprits), or escape of water from defective drains softening the sub-soil. The fix is to take the foundation down to stable sub-strata below the affected zone. (2) Basement conversion — creating a basement or lowered floor requires excavating below the existing foundation level, which means underpinning the entire perimeter first so the existing walls don't undermine. (3) Structural enhancement — when adding a double-storey rear extension or significant load to an existing structure, the original Victorian shallow footings may be inadequate. The engineer specifies whether mass-concrete underpinning, traditional pin-and-bay, or mini-piled solutions are appropriate based on ground investigation.
Mass concrete bay underpinning — the standard London method
Mass concrete bay underpinning is the most common method in London residential settings. The process: the existing footing is divided into a sequence of bays (typically 1m wide, alternating, never adjacent bays open simultaneously). For each bay: excavate by hand under the existing footing to the new depth (typically 1.5–3m below original founding); cast a steel needle to support the footing temporarily if span demands; pour mass concrete (Class C25/30) into the excavated bay, dry-packing the top 50mm to ensure full contact with the underside of the existing footing. Each bay is allowed to cure before the next adjacent bay is opened — typically a 24-48 hour gap. Total project duration scales with linear metres of underpin: a typical full-house basement conversion has 25–40 metres of perimeter underpin and takes 6–10 weeks.
Subsidence repair: insurance and remediation
Subsidence cracking is typically covered under buildings insurance. The standard process: (1) Damage spotted — vertical cracks wider than 3mm, doors/windows sticking, visible settlement. (2) Insurance claim opened — insurer instructs a structural engineer for diagnosis. (3) Investigation — typically 3–6 months of monitoring (crack gauges, level surveys) plus a soil investigation to confirm cause. (4) Repair specified — may be underpinning (for active subsidence), root barrier and soil moisture management (for tree-root drying), or just crack repair (if movement has stabilised). (5) Repair execution — Builderr is a specialist contractor on insurance subsidence works for major UK insurers. We handle the technical works and co-ordinate with the loss adjuster. Excess on a subsidence claim is typically £1,000; the underpinning itself is usually fully covered.
Party wall implications
Underpinning almost always triggers the Party Wall Act because the affected foundation typically borders or is near a neighbour's property. Section 6 of the Act (excavation notices) requires notice if you are excavating within 3m of a neighbour's structure to a depth below their foundations, or within 6m below a 45-degree line from their foundations. London terraces are tightly spaced so the 3m and 6m envelopes nearly always catch the adjoining property. We serve notices at design completion; the standard 1-month notice period applies for excavation. If the neighbour dissents, surveyors are appointed and an Award prepared specifying the working method, monitoring requirements and protection. Underpinning Awards typically include condition monitoring of the neighbour's property (crack gauges, level monitoring) for the duration of works.
