Subsidence is the downward movement of the ground supporting a building. Heave is the opposite: the ground swelling and pushing upwards. On clay soils both can happen at the same property in different years.
How shrink–swell works
Many soils contain clay minerals that absorb water and swell when wet, then lose water and shrink as they dry. BGS explains that how much a soil moves depends on the type and amount of clay it contains and on seasonal changes in moisture, driven by rainfall, drainage and vegetation.
Movement is greatest in the top few metres, which is why houses on shallow foundations are most affected, and it peaks in late summer and autumn after long dry spells. Clay-rich formations such as the London Clay, Gault, Oxford Clay, Kimmeridge Clay and Lias are among the most susceptible, which is why much of London and the south-east of England is exposed.
A growing risk
On 11 June 2026 BGS released a new version of its GeoClimate Shrink–Swell dataset, which combines 1:50,000 geological maps with climate projections up to 2070. It estimates that by 2070:
- about 1.8 million British properties (5%) could be highly or extremely likely to be susceptible to clay shrink–swell under a medium emissions scenario (RCP4.5)
- just over 4.2 million (11%) under a high emissions scenario (RCP8.5)
- around 500,000 under a low emissions scenario (RCP2.6)
- in London, over 26% of properties under medium emissions and up to 54% under high emissions
Northern and central London boroughs such as Camden, Islington and Barnet, and Kent, are among the most susceptible areas. BGS also cites Milliman UK figures showing domestic subsidence claims of £307 million in 2025.
The role of trees
Trees and large shrubs draw water from the soil through their roots. On shrinkable clay, a thirsty tree close to a house can dry the ground under the foundations far more than the weather alone. Species, size and distance all matter. See trees and foundations for the NHBC guidance on water demand and zones of influence.
Don’t fell a large tree without advice. Removing a mature tree on clay can let the soil rehydrate and swell for years afterwards (heave), which can also damage foundations. Trees may also be protected by a Tree Preservation Order or conservation area, so check with your council first.
Other causes of subsidence
- Leaking drains and water pipes, which soften or wash away soil beneath foundations
- Soft ground such as peat and alluvium, which compresses under load
- Sinkholes and old mine workings (see sinkholes)
- Landslides on slopes
Signs of subsidence
- new diagonal cracks, often wider at the top, around doors and windows
- cracks that can be seen inside and outside the same wall
- doors and windows that start to stick
- wallpaper rippling or gaps opening at skirting boards
- cracks that open in summer and close in winter, typical of clay movement
Hairline cracks are common in new plaster and as buildings settle. Cracks wider than about 3 mm, cracks that taper, or cracks that keep growing deserve an engineer’s inspection.
What to do
- Record crack widths and dates, with photos, so movement can be tracked.
- Tell your buildings insurer early. Most policies cover subsidence, usually with a higher excess.
- Have drains checked by CCTV survey if a leak is possible.
- Get a structural engineer to diagnose the cause before any repairs.
- If buying, ask about past claims and repairs, and commission a survey and environmental search.