A hairline crack is not automatically a property horror story. Houses move a little as materials expand, shrink and settle – particularly older homes with decades of London weather, alterations and enthusiastic DIY behind them. But knowing the signs of subsidence in a house can stop a minor viewing niggle becoming a very expensive surprise after completion.
For buyers, the aim is not to diagnose the building while standing in the kitchen with a takeaway coffee. It is to recognise when something deserves proper investigation, ask the right questions and make an informed decision before contracts are exchanged.
What subsidence actually means
Subsidence occurs when the ground beneath a property loses support or moves downward, causing part of the foundations to sink. Because one area may move more than another, the building can become distorted. That movement is what produces the visible clues buyers tend to notice.
It is different from normal settlement, which can happen as a newer building beds in, and from heave, where soil swells and pushes foundations upwards. All three can cause cracking, so the word “subsidence” should not be casually attached to every crack in the plaster. It has a certain drama to it, but a surveyor needs evidence before reaching that conclusion.
In South East London, properties are often built on clay soils. Clay can shrink in dry conditions and swell when moisture levels rise. Nearby trees, leaking drains and changes to drainage can all affect the moisture in the ground. That does not mean every Victorian terrace near a mature tree is in trouble. It means context matters.
Signs of subsidence in a house to look for
Diagonal cracks around openings
The classic warning sign is a diagonal crack running from the corner of a window or door, often wider at one end than the other. Cracks through brickwork, rather than just a thin line in internal paint or plaster, deserve particular attention.
As a broad rule, cracks wider than around 3mm may warrant closer inspection, especially where they are diagonal, stepped through mortar joints, or visible both inside and outside. A pound coin is not a scientific measuring tool, tempting though it may be. Note the location, shape and width, then have it assessed properly.
A single crack may reflect local movement, previous alterations or thermal expansion. Several similar cracks in related parts of the building can be more significant. The pattern is often more revealing than one isolated blemish.
Doors and windows that no longer behave
A door that sticks in winter can simply be swollen timber. A window painted shut may be a period feature of sorts. However, doors and windows that have become visibly out of square, suddenly jam, or show uneven gaps around their frames can indicate structural movement.
Look at whether the issue appears alongside cracking. If a door is catching and there is a fresh diagonal crack above it, that is a more meaningful combination than a grumpy bathroom door on its own.
Cracks where an extension meets the original house
Extensions are common across South London, and they do not always move in exactly the same way as the original building. A vertical crack at the junction may be caused by differential movement, particularly where the extension has shallower or different foundations.
This is not necessarily a deal-breaker. It may be old, stable and previously repaired. But a buyer should establish when the extension was built, whether building regulations approval was obtained and whether the crack has changed over time. Fresh filler and paint can be lovely. They can also be very good at keeping secrets for one viewing.
Sloping floors and distorted finishes
Uneven floors can have innocent explanations. Older houses often have floors that are less level than a pool table, and suspended timber floors can dip due to age, wear or localised deterioration. What matters is whether the slope is pronounced, appears to be worsening, or sits alongside other movement indicators.
You may also see gaps between skirting boards and floors, cracks at wall-to-ceiling junctions, or wallpaper that has torn along a line. These are supporting clues, not a verdict. A good inspection considers the whole building, rather than building a case around one wonky shelf.
External brickwork and render movement
Outside, check for stepped cracks following mortar joints, displaced bricks, bulging walls or cracks that continue across different surfaces. Render can conceal the masonry beneath, so its condition must be interpreted with care. A rendered wall with a crack is not proof of subsidence, but it may limit what can be seen and justify further investigation.
Pay attention to recent patch repairs. Repairs are not inherently bad news – sensible maintenance is a good sign. The useful question is whether they were cosmetic or followed an identified structural issue, monitoring programme or insurance claim.
Common causes, and why they matter to buyers
Clay shrinkage linked to nearby trees is a frequent cause of subsidence. Tree roots do not usually force foundations apart like something from a cartoon. More often, they draw moisture from shrinkable clay, causing the soil to contract. The effect depends on the tree species, size, distance from the house, depth of foundations and local ground conditions.
Leaking drains are another potential cause. Escaping water can wash away fine soil or alter ground moisture, while defective gutters and downpipes may saturate areas beside the property. Drain defects are often repairable, but identifying them early matters. A drainage survey may be recommended where there is evidence of movement or persistent damp around below-ground drainage.
Poorly designed alterations, mining history in certain locations, and changes in ground level can also play a part. The key point is that the cause affects the solution. Pruning or managing a tree, repairing a drain and monitoring movement are very different propositions from underpinning. The latter is not the automatic next step, despite what property forums might suggest at 1am.
What to do if you spot possible movement
First, do not panic and do not ignore it. Tell your surveyor what you noticed, including anything the estate agent or seller has said about past repairs, insurance claims or monitoring. A RICS Level 2 Home Survey can identify visible concerns in many conventional properties. Where the house is older, altered, unusually constructed, visibly cracked or being bought as a renovation project, a RICS Level 3 Building Survey is usually the more suitable choice. It allows for a more detailed assessment of construction, defects and likely next steps.
A survey cannot always confirm active subsidence from a single inspection. Movement may need monitoring over time, and a structural engineer may be needed to advise on its likely cause and significance. Further investigations could include a drain survey, trial holes to establish foundation depth, or a specialist report. That may sound like a lot, but targeted investigation is generally cheaper than guessing.
Ask the seller, through the proper conveyancing process, whether the property has had subsidence, underpinning, structural repairs or an insurance claim. Request relevant paperwork, such as engineers’ reports, completion certificates, monitoring records and guarantees. Historic subsidence that has been properly investigated, remedied and accepted by insurers is not automatically a reason to walk away. Active or unexplained movement is a different conversation.
Your solicitor should also check the implications for buildings insurance. Some insurers may offer cover with conditions, while others may be reluctant depending on the history and repair details. If you need a mortgage, your lender may require further information too. Sort this before exchange, not when you are trying to book removal vans.
Should subsidence stop you buying the house?
Not always. Plenty of homes with a historic issue go on to be sound, insurable and perfectly liveable. Equally, a property with unresolved movement can involve uncertainty, delay and costs that do not fit every buyer’s budget or appetite for a project.
Your decision should reflect the evidence, the proposed remedy, insurance availability and the price. If further work is required, it may be possible to renegotiate, but only once you understand what the issue is. A vague fear of subsidence is hard to price. A clear report, a defined scope of work and proper professional advice give you something far more useful: options.
A home purchase should feel exciting, even if the paperwork has other ideas. If cracks, sticking joinery or uneven floors are making you hesitate, get clear independent advice early. The right survey will not manufacture drama where there is none, but it can make sure you know exactly what you are buying.
