Subsidence, quietly.

What causes subsidence

Subsidence occurs when the ground beneath a property sinks or shifts, causing foundations to move and the structure above to crack or deform. The principal natural causes in the UK are: clay soil shrinkage during dry periods, drainage failures that erode load-bearing ground, and proximity to large trees whose roots extract moisture from clay soils (citing British Geological Survey GeoSure Dataset, a national hazard assessment for six ground stability indicators).

Human-driven causes include historic mining — coal, tin, salt, and brine extraction left voids that can collapse decades after operations ceased. Leaking drains, poorly compacted fill ground, and groundwater changes from nearby construction are further triggers. The BGS GeoSure dataset scores each 1 km grid square for six distinct ground stability hazards: landslides, shrink-swell, rock leaching, compressible ground, collapsible deposits, and running sand.

Drought-driven subsidence spikes

Subsidence claims are not evenly distributed through time. Prolonged dry spells cause clay soils to shrink more than usual, and claim volumes spike sharply. The Association of British Insurers records spikes in 2003, 2018, and 2022 — each following a summer with significantly below-average rainfall (citing ABI Subsidence Statistics Annual Report, published annually by the Association of British Insurers).

The 2022 spike followed the hottest July on record in England. Claim volume increased by over 40% versus the five-year average according to ABI data. Properties on shrink-swell clay soils with mature trees in the garden were disproportionately affected.

High-risk soil types by region

The BGS Geology of Britain Viewer maps geological formations across the UK at surface level, allowing buyers and surveyors to identify which clay formation underlies a property (citing BGS Geology of Britain Viewer, publicly accessible at bgsmapapps.bgs.ac.uk).

Tree proximity and the 1H rule of thumb

Trees growing in clay soils extract significant volumes of moisture, causing local shrinkage beneath foundations. The NHBC Standards (Chapter 4.2 — Foundations) use a proximity formula where the minimum safe distance from a tree to a foundation is related to the mature height of the tree (citing NHBC Standards Chapter 4.2, Building Near Trees, current edition).

The commonly cited "1H rule" — that a tree should be no closer than its mature height to the building — is a simplified approximation of NHBC's species-specific tables. Oak, poplar, willow, and eucalyptus are the highest-risk species. A surveyor will assess tree species, estimated height, root spread, and soil type to determine whether tree proximity is a subsidence risk factor.

Insurance implications

The Association of British Insurers Subsidence Code of Practice (updated 2022) sets out minimum standards for how insurers should investigate and handle subsidence claims (citing ABI Subsidence Code of Practice 2022). Properties with a documented subsidence history face specific challenges at insurance renewal and upon sale.

Standard subsidence excesses are typically £1,000 per claim. Where a claim has been made, the insurer may apply a higher excess for subsidence, exclude previously affected areas, or decline to renew. Buyers should always ask whether any subsidence claim has been made in the last ten years — this question appears on the Law Society TA6 Property Information Form. A "yes" answer requires the seller to provide full claim and remediation history.

Historic mining and the Coal Authority

Underground mining creates voids that can collapse long after extraction ceases. In England, the Coal Authority maintains the definitive record of coal mining activity and can identify whether a property is in a "Development High Risk Area" — where coal mining operations are recorded and subsidence risk from historic workings is elevated (citing Coal Authority Mining Reports service, published by the Coal Authority under the Coal Mining Subsidence Act 1991).

Similar risks exist in former tin, copper, and salt extraction areas. In Cornwall and parts of Devon, the Geoscience Consultants risk register covers former metalliferous mining. In Cheshire, brine extraction has left leaching-prone ground. A specialist ground investigation report is advisable in any area with historic industrial extraction.

What Eavesdrop checks

For every property report, Eavesdrop checks: BGS GeoSure hazard ratings for the property postcode (shrink-swell, compressible ground, rock leaching, and three other indicators), Coal Authority Development High Risk Area status, soil type from the BGS Geology of Britain dataset, and tree proximity data from satellite imagery where large canopy trees are visible within 20 metres of the building footprint. Where any risk factor is elevated, the report names the source dataset and explains what the rating means in plain English.