How to check a house for subsidence
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Reviewed by the HouseCheckup editorial teamQuick answer
Check for subsidence in three passes. Look for diagonal cracks wider than about 3mm near doors and windows, visible on both faces of the wall and getting wider, with sticking doors or sloping floors. Screen the ground under the postcode for shrink-swell clay and former mining. Confirm with a RICS surveyor or structural engineer, who monitors movement before diagnosing.
The six-step check
1. Look for diagnostic cracks
Check walls near doors and windows for diagonal cracks wider than about 3mm that appear on both the inside and outside of the same wall. These openings are the weakest points and where subsidence cracks usually show first.
2. Test whether the cracks are moving
Photograph and date each crack, or bridge it with a paper tell-tale. Cracks that widen over weeks or months indicate active movement; stable cracks that have not changed for years are usually cosmetic.
3. Check doors, windows and floors
Note any doors or windows that have started to stick or drop out of square, rippling wallpaper, and floors that slope or skirting that has pulled away — secondary signs that support (or weaken) a subsidence read.
4. Screen the ground under the address
Run the postcode check below for shrink-swell clay, recorded landslides and former coal mining. A low regional class or no returned record cannot diagnose a crack or rule out movement; a high one tells you to take the cracks seriously.
5. Ask the seller and check the paperwork
The TA6 Property Information Form asks the seller to declare known subsidence and past claims. Ask for underpinning records, structural-engineer reports, monitoring results and the buildings-insurance claims history.
6. Get a professional assessment
A RICS Level 2 or Level 3 survey, or a structural engineer's report, classifies the damage against BRE Digest 251 and advises whether monitoring is needed. This is the step that confirms or rules out subsidence.
What are the signs of subsidence in a house?
No single sign proves subsidence — surveyors look for a pattern. The table describes each common sign, what it actually looks like, and how much weight to give it.
| Sign | What it looks like | Concern |
|---|---|---|
| Diagonal cracks near doors and windows | Cracks running at roughly 45°, usually starting at the corner of a door or window opening (the weakest point in a wall). Subsidence cracks are typically wider than 3mm, wider at the top than the bottom, and visible on both the inside and outside of the same wall. | High |
| Cracks that appear suddenly and keep widening | New cracks that open up over weeks or months, rather than fine hairline cracks that have been stable for years. Progressive movement is the single strongest signal that ground movement is active. | High |
| Doors and windows sticking or jamming | Doors and windows that catch, drop or no longer close squarely, because the frame has been pulled out of square by foundation movement. Often noticed alongside cracking above the same opening. | Medium |
| Wallpaper wrinkling or rippling at wall/ceiling joints | Creasing or rippling of wallpaper where it is not caused by damp — commonly at the junction between wall and ceiling as the structure distorts. | Medium |
| Sloping or uneven floors | Floors that noticeably slope, or skirting boards that have pulled away from the wall, indicating part of the structure has dropped. | Medium |
| Gaps around extensions, bay windows or porches | Cracks or gaps where a newer, more lightly founded structure meets the main house — a common place for differential movement to show first. | Watch |
Concern is indicative only. Any crack wider than about 3mm, or any crack that is growing, warrants a professional opinion.
What do subsidence cracks look like?
Almost every home has some cracks. Subsidence cracks are diagonal, tapered and progressive; the table shows how each feature differs from an everyday cosmetic crack.
| Feature | Subsidence crack | Cosmetic crack |
|---|---|---|
| Width | Wider than about 3mm (thicker than a £1 coin edge); often 5mm or more | Hairline to ~1mm |
| Direction | Diagonal, typically running at roughly 45° | Straight, vertical or along a plaster join |
| Location | Starts at the corner of a door or window opening — the weakest point in a wall | Mid-wall, random, or where old and new plaster meet |
| Shape | Tapered — wider at the top than the bottom | Roughly even width along its length |
| Inside and outside | Visible on both faces of the same wall, often lining up | Usually only on the internal plaster surface |
| Over time | Progressive — keeps widening over weeks or months | Stable — unchanged for years |
When one part of a foundation drops, the wall above it rotates slightly around the point that stays put, which opens the crack more at the top than the bottom. Heave — the ground swelling upward, often after a large tree is removed and clay rehydrates — tends to do the opposite, so a crack that is wider at the bottom can point to heave rather than subsidence. A crack caused by ground movement usually passes through the full thickness of the wall, so it shows on the external brickwork and the internal plaster of the same wall; a crack that shows only on internal plaster is more likely thermal movement or plaster shrinkage.
“Cracks caused by subsidence are usually diagonal and wider at the top than the bottom. They often appear suddenly and are visible both inside and outside the property.”
How to read crack width: the BRE Digest 251 scale
UK surveyors classify structural cracking against BRE Digest 251, the Building Research Establishment's standard for assessing damage in low-rise buildings, from category 0 (negligible) to category 5 (very severe). Use it as a guide to how seriously to take a crack — width alone is not a diagnosis.
| Category | Typical width | What it means |
|---|---|---|
| 0 — Negligible | Up to 0.1mm | Not significant |
| 1 — Very slight | Up to ~1mm | Fine cracks, easily filled |
| 2 — Slight | Up to ~5mm | Cracks noticeable; filling and redecoration |
| 3 — Moderate | ~5–15mm | Repointing / some brickwork replacement; doors may stick |
| 4 — Severe | ~15–25mm | Extensive repair; walls lean or bulge |
| 5 — Very severe | Over ~25mm | Major work, possibly partial rebuild; structural instability |
Widths are indicative and follow the BRE Digest 251 classification used in RICS surveys. Category 2 and above near door and window openings warrants a professional assessment.
How do I check the ground under the address?
Most subsidence in England and Wales is caused by shrink-swell clay: clay-rich soils that swell when wet and shrink in dry spells, moving the foundations they support. The British Geological Survey describes shrink-swell as one of the most damaging geohazards in Britain, and the risk rises in hot, dry summers — the ABI reports claims spiking in drought years. Former coal mining and natural dissolution of soluble rock are the other common causes. Enter the postcode to see the clay class, recorded landslides and coal-mining area status for the area, on screen and with no signup. It is an area screen at the dataset's own scale, never a diagnosis of one building.
Why can't I confirm subsidence on my own?
Subsidence is diagnosed by movement over time, not by a single crack. A structural engineer or RICS surveyor rules out other causes — thermal cracking, wall-tie failure, lintel problems and heave — and monitors the cracks, sometimes across a full seasonal cycle because shrink-swell clay moves with the weather. Insurers and lenders act on that professional assessment, classified against the BRE Digest 251 scale, not on a homeowner's judgement. Confirming active subsidence usually takes months of monitoring; an initial survey can flag the likelihood quickly.
“If you are worried that your property may be suffering from subsidence, contact your buildings insurer, who will be able to arrange for the damage to be assessed.”
What does a Level 3 survey or a structural engineer add?
A RICS Level 2 survey notes visible cracking and recommends further investigation; a Level 3 (building) survey inspects more thoroughly and advises on repair; a structural engineer diagnoses the cause, specifies monitoring or trial pits and signs off remedial work. Which one you need depends on what the first two passes found — read our guide to a subsidence survey vs a structural engineer, and what underpinning costs if work is needed.
What should I do before buying a house I think has subsidence?
Ask for the seller's information and any past investigation, repair, monitoring and insurance documents. Agree the appropriate survey and any specialist investigation with a surveyor. Discuss property-specific searches with your conveyancer, including a coal mining search where the postcode is in a coalfield, and obtain insurance terms for the actual home — a previous claim can affect future cover. If you already own the home and cracks are actively widening, contact your buildings insurer; subsidence is a standard peril on most policies and the insurer will usually arrange monitoring. See buying a house with a subsidence history.
Take the ground picture to the surveyor
A crack tells you movement may be happening; the ground data tells you whether the address was ever at risk in the first place. The report screens the ground for any address in England, Wales and Scotland and sits it alongside flood, radon and coal mining, then names every check it could not run. Lite (£9.99) is the hazard screen; Complete (£24.99) adds the IQ Score and what stands out, the sold prices and EPC where the nation publishes them, planning labels and the questions to put to your surveyor, written from what the address flagged.
What the Complete report adds on ground stability for this address
Get the ground-stability findings — £24.99One-off · no subscription · Human-checked and emailed to you · see an illustrative sample
- The ground & subsidence panel for the address — coal mining, shrink–swell clay, recorded landslides and radon as traffic lights, with the six BGS GeoSure hazards rated for the grid cell and the distance to the nearest higher-rated cell
- The IQ Score, where subsidence, coal and radon together carry 46 of its 100 points, with the per-factor breakdown behind it
- What stands out at this address — the red and amber flags pulled together and explained
- The questions to put to your surveyor, written from what this address flagged — movement, mining and radon questions where they apply
Area ratings describe a grid cell, not the condition of one building, and do not replace a structural survey or a Coal Mining Risk Assessment.
Frequently asked questions
Related guides
Sources
- GeoSure — ground stability data — British Geological Survey (BGS)
- Shrink–swell: the most damaging geohazard in Britain — British Geological Survey (BGS)
- Subsidence — advice for homeowners — Association of British Insurers (ABI)
- Home surveys — levels and what they cover — RICS
- Get a coal mining report — Mining Remediation Authority / GOV.UK
HouseCheckup screens area-level official data to flag risk; it is not a structural survey and cannot confirm subsidence at a specific property. Where signs are present, commission a RICS survey or structural engineer. Crack descriptions and the BRE Digest 251 categories are general guidance, not a diagnosis.