A thin diagonal crack appears above a window. You notice it on a Sunday, decide it is the house settling, and forget about it for eight months. By the following spring it is wide enough to slot a £1 coin into, the door beneath it sticks, and a surveyor is using the word "subsidence" in a tone that makes your stomach drop. That sequence plays out in thousands of UK homes every year, and the gap between a £400 monitoring exercise and a £20,000 underpinning job often comes down to how early the owner reacted and how well they understood their buildings insurance.
Subsidence is the downward movement of the ground beneath a property, which drags the foundations with it. It is not the same as a house "settling" in its first few years, and it is not the same as heave, where the ground swells upward. For insurance purposes the distinction matters enormously, because standard buildings policies cover subsidence as a named peril — but they hedge it with an excess that dwarfs everything else on the schedule, and a claim leaves a mark on your property that outlasts the repair itself.
What subsidence actually looks like — and what it doesn't
Most cracks in a British home are cosmetic. Plaster shrinks, new extensions bed in against older brickwork, and seasonal humidity opens and closes hairline fractures that never threaten the structure. The cracks that should worry you have a particular signature. They are usually wider than 3mm — roughly the thickness of a 10p coin's edge — and they tend to be diagonal rather than vertical, running outward from the corners of doors and windows where the masonry is weakest. They are often wider at the top than the bottom, they appear on both the inside and outside of the same wall, and they show up alongside other symptoms: doors and windows that suddenly jam, wallpaper that rucks at the joints, or a visible lean in an external wall.
The cause underneath all this is nearly always water, or the lack of it. The single biggest trigger in the UK is clay soil shrinking during a dry summer, which is why a run of hot years pushes subsidence claims sharply higher — the Association of British Insurers recorded a major spike after the 2018 and 2022 droughts, with claim values running into the hundreds of millions in the worst years. Trees make it worse: a mature oak or willow within striking distance of the foundations can pull thousands of litres a day out of the soil, and the species and distance of nearby trees is one of the first things a loss adjuster will photograph. Leaking drains are the other classic culprit, washing fine soil out from under the footings until the ground gives way.
So what about old houses, the ones that have stood crooked and cheerful since the 1880s? An ancient cottage with sloping floors and a wonky door frame is usually showing historic movement that stopped settling a century ago — not active subsidence. The test is whether the movement is progressive. A crack that has been the same width since you moved in is part of the house's character. A crack that visibly grew over a single season is a problem that needs a professional eye now, not next year.
What your buildings insurance covers — and the catch nobody mentions
Subsidence is a standard peril on virtually every UK buildings policy, which sounds reassuring until you read the excess. While a typical claim excess sits around £100 to £250, the subsidence excess is fixed by regulation-era convention at a minimum of £1,000, and many insurers set it higher. That figure is per claim, and it applies before the insurer pays a penny — so a £6,000 repair to a bay window leaves you funding the first £1,000 yourself regardless of how diligently you have paid your premiums.
The cover itself is genuinely valuable, though, and it is worth being precise about what it includes. A subsidence claim that the insurer accepts typically pays for the investigation (soil tests, drain surveys, crack monitoring over several months), the remedial work to stabilise the foundations — which can mean underpinning, resin injection, or simply removing an offending tree — and the cosmetic repairs to walls, decoration, and floors once the ground has stopped moving. What it will not stretch to is improvement: if your 1930s footings were always marginal, the insurer fixes the damage but does not upgrade the whole house to modern foundation standards. There is a meaningful difference between "made good" and "made better", and that line is where a lot of disputes live.
One edge case trips people up repeatedly. If the cause is a leaking drain, the repair to the drain itself may fall under a different section of the policy — or not be covered at all if it is deemed wear and tear — even while the subsidence damage it caused is covered. You can end up with the insurer paying £15,000 to underpin a corner while telling you the £900 drain repair that caused it is your own bill. Read the policy wording on "gradual deterioration" before you assume everything flows from one claim.
Making a claim without sabotaging your own house
Here is the uncomfortable truth that estate agents know and most homeowners discover too late: a recorded subsidence claim follows the property, not just the policy. Once underpinning or a subsidence claim is on the record, you are obliged to declare it to future insurers and to buyers, and it can make the house harder to insure and slower to sell for years afterwards. That does not mean you should hide a serious structural problem — that way lies a far bigger disaster — but it does mean you should be deliberate about when a claim is genuinely the right move.
If you see the warning signs, the sensible order of events looks like this:
- Photograph the crack with a ruler or coin against it for scale, dated, so you can prove whether it is growing.
- Check for the obvious non-structural causes first — a blocked gutter dumping water against one wall, or a recently removed tree that changed the soil moisture.
- Get an independent opinion from a structural engineer (RICS-registered, expect £300–£600) before you phone the insurer, so you walk into the conversation knowing whether this is monitoring or underpinning.
- Only then notify your insurer, who will appoint their own loss adjuster and almost certainly start with a monitoring period of six to twelve months rather than rushing to dig.
That monitoring period frustrates people, but it is the right call. Underpinning is the nuclear option — it is expensive, disruptive, and it is the single fact most likely to scare off a future buyer. A good insurer will exhaust the gentler fixes first: managing the trees, repairing drains, and watching whether the movement stabilises once the underlying cause is dealt with. Insist on a copy of every monitoring report, because if you sell before the claim is closed, those documents are what a buyer's surveyor will want to see.
Is it even worth claiming?
Sometimes the answer is no.
If a structural engineer tells you the crack is minor, non-progressive, and fixable with filler and a coat of paint for a few hundred pounds, claiming is almost always the wrong move. You would burn your £1,000-plus excess to fund a repair you could pay for outright, and you would saddle the property with a declarable subsidence history for the privilege. The maths only tips toward a claim when the engineer's verdict involves real foundation work — underpinning, resin injection, significant drain excavation — where the bill runs well into five figures and self-funding stops being realistic. A useful rule of thumb: if the quote to fix it comfortably exceeds your excess and the cause is active rather than historic, claim; if it is a one-off cosmetic repair, pay it yourself and keep your record clean. There is no shame in treating your insurer as the backstop for the catastrophic job rather than the handyman for the small one — that is precisely what subsidence cover is designed to be.
Keeping your premium and your home protected
Prevention is unglamorous and it works. The cheapest subsidence insurance is a well-maintained drainage run and a sensible relationship with the trees around your plot. Keep gutters and downpipes clear so water is carried away from the foundations rather than soaking into the clay beside them. Do not plant thirsty species — willow, poplar, oak — within about their mature height of the house, and think twice before felling a long-established tree in a dry year, because removing a big tree from clay soil can cause heave as the rehydrated ground swells back up, which is its own expensive headache.
On the insurance side, never let a subsidence history tempt you into hiding it, and never assume switching insurer at renewal will be simple once a claim is on record — many of the cheapest comparison-site quotes quietly exclude subsidence or load the excess for properties with any history of ground movement. If you live in a high-risk postcode on shrinkable clay — much of London, the South East, and the Midlands sits on it — it is worth checking your renewal documents specifically for the subsidence excess and any new exclusions, because insurers adjust these quietly year to year and the change rarely arrives with a flashing warning.
The homeowners who come out of this well are not the ones who never get a crack. They are the ones who photographed it the week it appeared, knew the difference between cosmetic and structural, and made the insurer earn the gentler fix before anyone reached for a digger.