Subsidence: surveyor, structural engineer or property report?

Why subsidence deserves a plan, not a guess

The British Geological Survey calls clay shrink–swell “the most damaging geohazard in Britain today”, and records that it cost the economy £540 million after the 1991 drought. The clay formations most prone to it sit mainly in the south-east of Britain. Clay soils swell when wet and shrink when dry; when that change is uneven beneath the foundations — often because a nearby tree is drawing water from one side — the building moves and cracks. Leaking drains that soften the ground and former mine workings are the other common causes.

Not every crack is subsidence. Plaster shrinkage, thermal movement and settlement that stopped years ago are common findings too. The point of using three different professionals is to separate the ordinary from the expensive without paying for every one of them on every house.

Stage one: screen the ground before you offer

A pre-offer property report reads the ground hazards for the address from national datasets: shrink–swell clay, compressible and collapsible ground, landslide, running sand, soluble rock, radon and coal-mining areas. Be clear about what that is. The geological survey's hazard ratings are area ratings — each cell covers roughly 65 square kilometres — so they describe the ground in the area, not your plot. A “significant” clay rating does not mean the house is moving, and a “low” rating does not clear a house with fresh cracks.

What the screen gives you is direction. On low-hazard ground with no disclosed history, a standard survey is usually proportionate. On clay with large trees close to the house, or in a former coalfield, you want a surveyor who will look for movement deliberately — and you want to know before you have paid for anything else. Our ground stability check shows the area picture for a postcode.

Stage two: the RICS survey

A RICS Home Survey is a visual inspection of the whole building by a chartered surveyor. On movement, a good surveyor looks at the pattern of cracking — diagonal cracks that widen towards the top, cracks that show inside and out, distorted openings — and at evidence of past repairs. The survey does not dig, monitor or test drains. When the surveyor sees something that needs a specialist, the report says so, and that recommendation is the trigger for stage three. Choose Level 3 for older, altered or unusual buildings; our report-vs-survey guide covers the levels and indicative costs.

Stage three: the structural engineer

A chartered structural engineer — look for MIStructE or MICE after the name — investigates whether a building is moving, why, and what will stop it. On a purchase that usually means an inspection and a written report on the cracking. On a house with active movement it can mean monitoring cracks over a period of months, trial pits to see the foundations and the soil, a drain survey, and advice on nearby trees. HomeOwners Alliance gives £500–£2,000 for a structural engineer's report and £90–£200 an hour; the spread is the difference between one crack and a full investigation.

If the house has had a subsidence claim, the most valuable documents may already exist: the insurer's engineer's report, a certificate of structural adequacy after repairs, and any underpinning guarantee. Ask the seller for them before you pay for your own engineer — then decide whether you need an independent opinion on top.

Insurance is the test that settles it

Whatever the reports say, the practical question is whether you can insure the house at a price you can live with. A past claim can raise premiums and excesses and tie you to the existing insurer. Get a buildings insurance quote on the specific address while the purchase is still reversible; a house nobody will insure is a house your lender will not lend on.