What are the signs of subsidence? (UK guide 2026)

The short answer

The classic signs of subsidence are diagonal cracks that are wider than about 3mm (thicker than a 10p coin edge), tapering so they are wider at the top than the bottom, and appearing around doors and windows. They often run through the mortar and the brick, show up both inside and outside in the same place, and may be accompanied by doors and windows that suddenly stick, rippling or torn wallpaper at wall junctions, and gaps opening where extensions or bay windows meet the main house. Subsidence cracks usually appear fairly suddenly and keep growing, often worse after a hot, dry summer. Fine, stable hairline cracks, and cracks confined to plaster alone, are far more likely to be ordinary settlement or seasonal movement than subsidence.

Most cracks in a home are harmless, so the goal is to tell the worrying signs from the everyday ones. The sections below set out what genuine subsidence looks like, where it tends to appear, and the practical checks that help you judge how serious a crack really is.

Tell-tale signs

Crack width that mattersWider than ~3mm and growing
Typical directionDiagonal, often from corners of openings
WhereDoors, windows, extension joints
Worse afterHot, dry summers
First callBuildings insurer or a RICS surveyor

The signs that genuinely point to subsidence

Subsidence is the downward movement of the ground beneath your foundations, which pulls part of the structure with it. Because the movement is uneven, it tends to leave a recognisable set of clues rather than a single crack:

Diagonal cracks wider than 3mm that taper — wider at one end — and often start at the corner of a door or window where the structure is weakest.
Cracks visible inside and out in roughly the same place, running through the brickwork and mortar rather than just the surface plaster.
Doors and windows that begin to stick or no longer close squarely, because the frame has been pulled out of true.
Rippling, creasing or tearing wallpaper at the junction of walls and ceilings, where movement concentrates.
Gaps opening up where an extension, porch or bay window meets the main house, as the two parts move at different rates.

One sign on its own rarely confirms subsidence. It is the combination — a wide diagonal crack, plus a sticking door, plus the crack showing on both faces of the wall — that should prompt a professional assessment.

Signs that usually are NOT subsidence

It is just as important to recognise the harmless movement that affects almost every UK home. Mistaking normal settlement for subsidence causes a lot of needless worry and, occasionally, unnecessary insurance claims.

Fine hairline cracks (under about 1mm) in plaster, especially near new plasterwork as it dries and shrinks.
Vertical cracks at the join between an older house and a newer extension, which often simply mark the meeting of two structures.
Cracks that appear once and then stay the same size for years — stable cracks are rarely a sign of active subsidence.
Stepped cracks confined to mortar in a boundary or garden wall, which is not load-bearing.

A simple monitoring trick: if you are unsure whether a crack is active, mark each end with a dated pencil line or fix a small piece of paper across it, then photograph it monthly. A crack that visibly widens over weeks or months is far more concerning than one that holds steady through a full change of seasons.

Where the signs are most likely to appear

Subsidence clues do not appear at random — they cluster where a building is structurally weakest and where the ground is most prone to moving. Knowing the hot spots helps you check the right places:

Around door and window openings: these interrupt the run of brickwork, so cracks radiate from their corners first.
At extension and bay junctions: where two structures with different foundations meet, differential movement opens gaps.
On the elevation nearest a large tree: on clay soil, the wall closest to a thirsty tree is the most exposed to seasonal shrinkage.
Near drainage runs: a wall above a leaking drain can move as the ground beneath softens or washes away.

Properties most at risk are older homes with shallow foundations, houses on shrinkable clay in London, the South East and East of England, and homes with mature trees or ageing drains close by. If your property ticks several of these boxes and you see the warning signs, that combination is worth a professional eye sooner rather than later.

What to do if you spot the signs

If you see several of the warning signs together, do not panic and do not start hacking out plaster. Subsidence is usually slow and rarely makes a house unsafe overnight. The sensible order is: photograph and measure the cracks, note when they appeared and whether they are growing, and contact your buildings insurer, since subsidence is a standard insured peril on most UK home policies. The insurer will normally arrange for a chartered surveyor or structural engineer to investigate the cause — which may turn out to be a leaking drain, a nearby tree drawing moisture from clay soil, or genuine foundation movement. Underpinning is only one of several possible outcomes, and it is the exception rather than the rule.

Frequently asked questions

How wide does a crack have to be to indicate subsidence?

As a rough guide, cracks wider than about 3mm (thicker than the edge of a 10p coin) that are diagonal and growing warrant investigation. Surveyors often use the BRE crack-width categories, where cracks above 5mm are classed as serious and above 15mm as severe. Width alone is not proof — direction, location and whether the crack is still moving all matter.

Does subsidence happen more in summer?

Cracks linked to subsidence often appear or worsen after a hot, dry summer, particularly on shrinkable clay soils that shrink as they dry out. Many homeowners first notice the signs in late summer or autumn, and some movement can partly reverse over a wet winter.

Can I tell if it is subsidence myself?

You can spot the warning signs and monitor whether cracks are growing, but you cannot confirm the cause without investigation. Only a chartered surveyor or structural engineer can establish whether the ground is moving and why, usually after inspecting the property and sometimes digging trial holes to examine the foundations and soil.

Sources & further reading

Figures on this page are typical UK ranges drawn from published sources and depend on your specific property. They are guidance, not a quotation.