If there's one word in a building survey report that sends buyers into immediate panic mode, it's "subsidence". But let me share something important right at the start: the majority of cracking in properties that might look like subsidence isn't subsidence at all. And even when it is, it doesn't necessarily mean you should walk away.
What Is Subsidence?
Subsidence occurs when the ground beneath a building's foundations moves downward, causing the foundations to sink and the building to move with them. When this movement is uneven — affecting one part of a building more than another — it creates differential settlement, which is what typically causes the cracking that alerts buyers and surveyors to a potential problem.
It's important to distinguish between subsidence (the ground moving downward) and heave (the ground moving upward) — both can cause structural damage but have different causes and remedies.
Common Causes of Subsidence in West Sussex
In our experience of surveying property across Crawley and West Sussex, the most common causes of subsidence and significant structural movement are:
Tree Root Activity
Tree roots — particularly from mature trees like oaks, poplars, willows and limes — can extract moisture from clay soils, causing the soil to shrink and the foundations to drop. This is one of the most common causes of subsidence in West Sussex, where many older properties have large mature trees in close proximity.
Shrinkable Clay Soils
Much of West Sussex sits on Weald Clay or other shrinkable clay soils, which expand when wet and contract when dry. Properties on these soils are inherently more vulnerable to seasonal movement, particularly during drought periods when soils dry out significantly.
Leaking Drains
Leaking underground drains can wash away the soil beneath foundations (a process called soil wash), undermining the foundations and causing localised settlement. This is why we recommend CCTV drainage surveys alongside structural investigations in older properties.
Mining and Geological Factors
Parts of West Sussex have historic mining activity. It's always worth checking historical land use around any property you're buying.
Signs of Subsidence to Look For at Viewing
- Diagonal cracking running from the corners of windows or doors — particularly if wider at the top than at the bottom
- Cracking that goes through both brick and mortar (as opposed to following the mortar joints only)
- Cracks wider than 3mm — as a rough guide, a crack wider than a 10p coin is worth investigating
- Doors and windows sticking suddenly, particularly if recent, which can indicate frame distortion
- Sloping or uneven floors — though this is common in older properties and doesn't necessarily indicate active movement
What It Means If Your Survey Flags Subsidence Risk
A good survey report will distinguish between: (1) cracking that requires monitoring but is likely historic and stable; (2) cracking that requires specialist structural investigation; and (3) cracking that indicates likely active subsidence requiring immediate action.
If your surveyor recommends further investigation by a structural engineer, that's not necessarily bad news — it's the right call to get clarity. A structural engineer's report might well confirm that the movement is historic, stable, and not a cause for concern. Or it might confirm active subsidence, in which case you need to understand the remediation options and costs before deciding how to proceed.
Subsidence and Property Insurance
Properties with a history of subsidence can be more challenging to insure. Always check whether the seller has made any insurance claims for subsidence, and ask for documentation of any previous underpinning or remediation work. Your solicitor should raise these questions as part of their enquiries.
"Don't panic when you see cracks. Understand them. Most cracking has a benign explanation. But you need an experienced eye to tell the difference — and that's exactly what we're here for."
If you're concerned about a specific property's structural condition, contact our team to discuss a structural survey.