The short answer
A building survey will identify the visible signs of structural movement — including subsidence — but it does not confirm the cause or whether movement is ongoing. The surveyor looks for diagonal cracking widening towards the top, cracks around door and window openings, distorted frames, sloping floors, separation between extensions and the main house, and movement near trees or drains. They assess crack width, pattern and location against recognised categories and form a view on whether it looks like historic settlement or active subsidence. Because confirming subsidence needs monitoring, trial pits or below-ground investigation that a non-intrusive survey cannot do, the report recommends a structural engineer where the evidence is significant. Cosmetic settlement cracks are common and not the same as subsidence.
Subsidence is a word that frightens buyers. Here is what a building survey can realistically tell you about it, and where the inspection has to hand over to an engineer.
Subsidence check at a glance
- Detects signs?Yes — visible movement
- Confirms cause?No — needs investigation
- Key clueCrack width, pattern, location
- Common triggersTrees, clay soil, leaking drains
- Next stepStructural engineer report
The signs a surveyor looks for
Subsidence shows as downward movement of the ground beneath the foundations, and the building reacts by cracking. A surveyor reads several clues together rather than a single crack: diagonal cracks that are wider at the top than the bottom, cracking that runs through brick and mortar (not just along joints), cracks radiating from window and door corners, sticking or distorted openings, sloping or springy floors, and gaps where an extension or bay pulls away from the main wall. They also weigh the setting — nearby large trees, shrinkable clay soil, historic mining, or leaking drains washing out fines all raise the suspicion. Crucially, they distinguish this from thermal and shrinkage cracking, which is hairline, follows joints and is cosmetic.
Why crack width and pattern matter
Surveyors interpret cracking against established width categories, because a fine crack and a wide one mean very different things for cost and urgency. The figures below are widely used industry guidance for masonry, not a diagnosis on their own.
| Crack category | Approx. width | Typical implication |
|---|---|---|
| Negligible / hairline | Up to ~1 mm | Cosmetic, redecoration only |
| Slight | ~1–5 mm | Filling and monitoring |
| Moderate | ~5–15 mm | Repair, possible investigation |
| Severe | ~15–25 mm+ | Significant; engineer required |
Indicative crack categories used in UK practice; width alone is not proof. Source: BRE Digest crack categorisation, RICS.
Why the survey hands over to a structural engineer
A building survey is non-intrusive, so it cannot dig trial pits to see the foundations, cannot install crack monitors to prove whether movement is ongoing, and cannot run the soil and drainage tests that pin down the cause. When the surveyor sees moderate or severe cracking, a clear progressive pattern, or evidence of recent movement, the report recommends a structural (or geotechnical) engineer to investigate and advise on remedy — which can range from monitoring to drain repair, tree management or, in serious cases, underpinning. It is also worth checking the insurance position: a property with a subsidence history may have restricted cover or a higher excess, which a structural report and any past records will clarify.
Frequently asked questions
Can a building survey confirm a house is subsiding?
Not on its own. It identifies the visible signs and judges how serious they look, but confirming active subsidence and its cause needs monitoring, trial pits or soil tests done by a structural engineer, which the report will recommend.
Are cracks in a house always a sign of subsidence?
No. Most cracks are thermal or shrinkage movement — hairline, following joints, and cosmetic. Subsidence cracks tend to be diagonal, wider at the top, run through brickwork and cluster around openings. Surveyors distinguish the two.
Will subsidence affect my insurance or mortgage?
It can. A subsidence history may mean restricted cover, a higher excess, or a lender wanting a structural report before lending. Obtaining an engineer's report and any past repair records helps clarify the position before exchange.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on the specific property and survey level. They are guidance, not a quotation.