You spot a crack running up the wall above the bay window, and within about four minutes of searching you've decided the house needs underpinning. Most cracks don't mean that. Some do. The difference isn't guesswork, and surveyors have been using the same scale to make the call since 1995.
What subsidence actually is
The Royal Institution of Chartered Surveyors defines it plainly: subsidence is "the downward movement of the foundations of a building due to changes in the supporting strata." That's the ground underneath moving, not the building itself sagging under its own weight.
That distinction matters because several things that look identical on a wall are something else entirely. Thermal movement, where materials expand and contract with temperature, opens and closes hairline cracks with the seasons. Shrinkage in freshly applied plaster does the same in a newly decorated room. Settlement in a recent extension is normal for the first year or two as new foundations bed in. A deflecting lintel over a door or window can crack the wall above it without anything underneath having moved at all. RICS is explicit that subsidence is different from settlement, and conflating the two is most of the panic.
The scale surveyors actually use
BRE Digest 251, revised in 1995, is the classification structural engineers, insurers and surveyors still reach for. It groups damage into six categories by width, and the grouping is the part worth remembering:
| Category | Typical width | Class |
|---|---|---|
| 0 | Hairline, under 0.1 mm | Negligible |
| 1 | Up to 1 mm | Very slight |
| 2 | Up to 5 mm | Slight |
| 3 | 5–15 mm | Moderate |
| 4 | 15–25 mm | Severe |
| 5 | Over 25 mm | Very severe |
Categories 0 to 2 are aesthetic. They need redecoration and nothing else. Categories 3 and 4 are serviceability issues: they affect how weathertight the building is and whether doors and windows still operate properly. Category 5 is a stability issue, and it's likely to need structural intervention.
BRE's own guidance adds a caveat worth taking seriously: classifying damage on width alone can be misleading. A single fine crack and a dozen 3 mm cracks across a wall can add up to the same category despite looking very different from across the room. Pattern and how a crack changes over time tell you more than one measurement does.
What actually causes it
Most UK subsidence comes down to one of three things.
Clay shrinkage is the big one. Clay soil shrinks when it dries out and swells again when it's wet. A long dry spell pulls moisture out of the clay under a house, the ground shrinks, and the foundation drops with it. Trees are the usual reason the clay dries out, not the weather alone. Which? estimates that around 70% of subsidence cases involve tree roots drawing moisture out of a shrinkable soil, which is why a large, thirsty tree planted close to a house on clay is the classic setup.
Drains and water mains are the second cause. A leaking pipe softens the soil around it or washes it away entirely, particularly in sandy or gravelly ground, and the foundation loses support from underneath.
Mining subsidence exists but is rare outside former coalfield areas, where old workings can collapse decades after they were dug.
The cracks worth worrying about
RICS is specific here: "there is rarely any cause for real concern unless cracks appear suddenly and are more than 3mm wide." Below that, you're almost always looking at cosmetic movement.
Above it, the pattern matters more than the width. Worth taking seriously:
- Diagonal, tapered cracks, wider at the top than the bottom
- Cracks that run through the brickwork itself, not just the render or the internal plaster
- Doors and windows that have started sticking or sit out of square
- Wallpaper rucking, or new gaps opening at the skirting boards
- Cracking near an extension, a bay window, or anywhere an older and newer part of the building meet
Worth naming so you don't panic over it: hairline cracks that open and close with the seasons, fresh cracking in new plaster, minor settlement in an extension built in the last year or two, and a crack that stops at a lintel rather than continuing into the brickwork above. Damp can produce cracking too, particularly where rising or penetrating damp has softened plaster or masonry, so a crack next to a damp patch is worth diagnosing as one problem rather than two.
If your insurer needs to know
Notify your buildings insurer as soon as you reasonably suspect structural movement, with photographs, dates, and details of anything that's changed recently, like a tree taken down or a new extension. Two things not to do: don't fill or paint over the cracks before anyone's inspected them, because that's the evidence.
What follows is slower than most people expect. Movement is seasonal, so RICS notes that cracks "may need to be measured and monitored, perhaps for as long as 12 months, or in extreme cases for longer" before an insurer will agree a remedy. That's not bureaucracy for its own sake. Confirming the ground has actually stopped moving before anyone repairs the wall is the whole point of the wait.
The reassuring part gets buried under the horror stories: RICS estimates that fewer than 5% of properties with subsidence need underpinning, and where it is needed, it typically costs somewhere between £10,000 and £75,000 for a normal house, more for something larger. Most cases resolve with removing or reducing a problem tree, or fixing a drain, both a good deal cheaper than digging out and replacing a foundation.
Buying a house with a subsidence history
A past subsidence claim, even a resolved one, follows the property rather than the seller. It often means the house can only be insured through the insurer that handled the original claim, or through a specialist, which narrows your choice of buildings insurance and can mean a higher premium. That's worth pricing in before you commit, not after.
Claims tend to succeed more easily where there's clear evidence of an actual insured event, a specific tree, a specific leaking drain, rather than what looks like ordinary ageing in an older building. If a seller mentions a historic claim, ask what caused it, what was done about it, and get a quote for buildings insurance before you exchange rather than assuming it'll be routine.
What to do about it
- Measure it and date it. Width in millimetres, and a photo with today's date next to it. You can't tell if a crack is moving without a baseline.
- Check the pattern, not just the width. Diagonal and tapered beats straight and uniform. Through the brick beats through the plaster.
- Look for a cause nearby. A large tree close to the house, a recently built extension, a drain run under the affected wall.
- Test doors and windows near the crack. Sticking or going out of square is a stronger sign than the crack alone.
- Ask the seller directly if there's been a claim. It's a normal question, and their answer affects your buildings insurance quote.
- If you're already worried, brief the surveyor before they attend. RICS's own terms tell surveyors to be told about a crack in advance so they can look at it properly, rather than hoping they notice it on the day. A Level 3 survey is the one that's actually asked to propose a cause, not just record what it sees.
- Get your own buildings insurance quote before exchange if there's any history at all, so you're not finding out about a restricted market after you're committed.
The honest summary
Most cracks are nothing: seasons, new plaster, a young extension settling in. The ones worth taking seriously are wider than about 3 mm, diagonal, running through brick rather than plaster, and getting worse rather than staying still. Even then, underpinning is the exception rather than the rule, and the process that follows is slow because it has to be: nobody can honestly tell you the ground has stopped moving after one visit.
The crack itself is rarely the story. What caused it usually is.
This is general information, not structural, legal or insurance advice. It describes the position in England and Wales. Guidance and figures as at August 2026. Get a chartered surveyor or structural engineer to look at any crack you're genuinely concerned about before you rely on anything in this post for a buying decision.