Subsidence in London Homes: Causes, Signs, and What to Do

Subsidence is a particular concern in London because of the combination of London clay soils, a history of mature tree planting close to properties, and a housing stock in which many foundations are shallow by modern standards. Understanding subsidence, its causes, and its symptoms allows homeowners and buyers to make informed decisions about properties that may be affected.

Subsidence in London is overwhelmingly caused by moisture loss from London clay — a shrink-swell soil that contracts when it dries out and expands when it absorbs moisture. The most common driver is tree root activity: mature trees, particularly certain species, extract large volumes of moisture from the soil and can cause the clay beneath foundations to shrink and subside. The most problematic species in the London context include oak, poplar, willow, elm, and plane trees, which have extensive lateral root systems that can affect foundations at distances of fifteen to thirty metres from the trunk.

Other causes of subsidence include leaking drains (which can wash fine particles out of the soil beneath foundations, creating voids), inadequate foundation depth, and in some parts of London, historic mining activity or buried structures.

The signs of subsidence include cracks in external brickwork that are wider than 3mm, diagonal in orientation, and wider at the top than the bottom — especially near doors, windows, and corners. Internal cracks following mortar joints diagonally or stepping in a staircase pattern are also characteristic. Doors and windows that suddenly become difficult to open or close, particularly in the same part of the house, can indicate movement. A structural engineer or specialist surveyor can distinguish subsidence cracks from normal settlement or thermal movement.

If you suspect subsidence, the first step is to commission a structural engineer's or RICS-qualified building surveyor's report. The report will assess the likely cause, monitor the cracks over time to determine whether movement is ongoing or historical, and recommend remedial action if required.

Insurance claims for subsidence are handled by building insurers. Notification to the insurer should happen promptly when subsidence is suspected — non-disclosure can affect coverage. Most insurers will appoint their own specialist to investigate and manage remedial works.

Remedial options depend on the cause. Where trees are the cause, the primary remedy may be tree removal or significant pruning, allowing the soil to rehydrate and the building to re-heave over a period of years. Underpinning (deepening the foundations) is used where soil movement is likely to continue or where tree removal is not feasible. Modern underpinning methods include traditional mass concrete underpinning and more modern resin injection and pile-and-beam systems.

RCB works with specialist structural engineers on subsidence investigation and remedial projects across London. +44 7359 872594 | connect@rcbGroup.co.uk | www.rcbGroup.co.uk