The crack runs from the corner of the bay window up towards the gutter, wider at the top, and the seller says it has "always been there". It might have. Old houses move, and most cracks are plaster shrinkage. But a particular kind of crack, in a particular kind of soil, after a particular kind of summer, is the start of a story that ends with a loss adjuster and a certificate of structural adequacy, and it helps to know the plot before you offer.
What are the signs of subsidence?
Subsidence is the ground under part of the house sinking, and that part going with it. The cracks are diagonal, stepped along the mortar joints, wider at the top than the bottom, and visible inside and outside at the same spot. Anything wider than about 3mm is where surveyors start paying attention.
Other signs are doors and windows that stick for no reason, wallpaper rippling where wall meets ceiling, and a gap opening where an extension meets the original house, because the two sit on foundations of different depths. A hairline crack above a window is usually not subsidence. A fresh diagonal crack near a big tree deserves a closer look.
Why do clay soil and trees matter?
Most subsidence in England is clay shrinkage. Clay swells when wet and shrinks when dry, and a large tree, especially an oak, willow or poplar within about ten metres of the house, pulls water out of the soil through its roots in a dry summer. The clay under that side shrinks, the foundation drops a few millimetres, and the brickwork cracks. The London clay belt, Essex, Kent and Hertfordshire are where most claims come from.
The timing is oddly predictable. A dry summer means cracks in autumn and claims in winter, and 2018 and 2022 both produced a surge. Houses built before the 1970s, when foundations were often less than a metre deep, are the most exposed.
What does subsidence do to insurance?
A subsidence claim stays with the house, not the owner. Buildings insurers ask whether the property has ever suffered subsidence, heave or landslip. Most policies already carry a £1,000 excess for subsidence claims. If the answer is yes, some insurers will decline, and others will charge more, raise the subsidence excess further or exclude subsidence altogether.
If the house has been underpinned, ask for the certificate of structural adequacy from the engineer. Under an Association of British Insurers agreement, the existing insurer will usually offer terms to the buyer of a property they have paid a subsidence claim on, which is often the easiest route.
How do I check the street, not just the house?
The British Geological Survey's GeoSure dataset maps six ground hazards, including shrink swell clay, and shows whether the postcode sits on high, medium or low potential soil. It is the layer behind most "subsidence risk postcode checker" tools, and the environmental search from your solicitor usually pulls in the same data as a ground stability report.
Then walk the road. Look for patches of new brickwork at the corners and large trees on the pavement. Check the planning register for applications to fell or prune protected trees, which often follow a claim. The neighbours will know who has claimed.
What should I ask?
Ask the seller: "Has the house or any neighbouring house had a subsidence claim, and has any work been done to the foundations or the drains?" Ask your surveyor for a RICS Level 3 survey if there is any cracking, a large tree close by or clay on the map, and ask them to say in writing whether the cracks look historic and stable or recent and progressive. Ask your solicitor: "Does the environmental search include a ground stability report?" And ask the seller's insurer, through the seller, whether they will offer you continuation of cover.
How Eavesdrop checks this
Eavesdrop reads the BGS GeoSure shrink swell layer, the Coal Authority mining reports where relevant, the planning register for tree works and underpinning applications, and Land Registry sold prices for the road, then cites each source. Try [check: subsidence] and [check: planning-next-door]. Before you offer. Before you sign. Eavesdrop.
Quick answers
What are the first signs of subsidence? Diagonal cracks wider at the top than the bottom, visible inside and outside, doors and windows sticking, and rippled wallpaper where walls meet ceilings.
Does subsidence affect house insurance? Yes. A subsidence claim follows the property, and future insurers may decline or charge a higher excess, so ask whether the current insurer will continue cover.
How do I check subsidence risk by postcode? The British Geological Survey GeoSure dataset maps shrink swell clay potential for any postcode, and a ground stability report from your solicitor's environmental search uses the same data.
Sources: British Geological Survey GeoSure dataset; Association of British Insurers guidance on subsidence and continuation of cover; RICS Home Survey Standard (Level 3); Building Research Establishment Digest 251 on the assessment of cracks in buildings.
