Subsidence: Is That Crack a Problem?

Image note: This image is AI-generated and intentionally exaggerated for illustration. Subsidence does not typically cause a property to sink this dramatically.

You're viewing a house, or looking at your own wall, and you spot a crack. Your stomach drops. Is it subsidence?

Here's the reassuring news first: most cracks are not subsidence. Homes move a little all the time: plaster shrinks, materials expand and contract with the seasons, and new-builds settle. Many older houses, Victorian terraces especially, have some movement in their history that happened decades ago and stopped long ago.

But subsidence is one form of movement you do need to take seriously. It can affect repair costs, insurance, mortgageability, and ultimately whether buying the property still makes sense. So it's worth knowing how to tell the difference.

What subsidence actually is

Subsidence is when the ground under your home sinks, and the foundations move down with it. Because the movement is usually uneven, it pulls the building out of shape, and that's what cracks the walls.

Two things it's often confused with:

Settlement, the small, natural sinking a new building does in its first few years. Normal.
Heave, the opposite: the ground swells and pushes the foundations up.

And not every structural crack comes from the ground at all. Problems with lintels, thermal movement, past alterations, extension junctions or roof spread can all crack a wall too.

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Movement isn't always active subsidence

This is the most important thing to understand, and it's where buyers panic unnecessarily. There's a big difference between:

Historic movement, the house shifted at some point, then settled and has been stable for years. Very common in older homes on clay. Often no action needed.
Active subsidence, the ground is still moving now. This is the one that needs fixing, because it gets worse until the cause is dealt with.

Victorian and Edwardian bay windows, for instance, often sit on shallower foundations and can lean forward slightly over the decades. It looks alarming, but it's frequently old, minor and long since stopped.

A survey often says “evidence of movement, structural engineer investigation recommended.” That is not a verdict. It's the surveyor flagging a question they can't answer just by looking.

The warning signs

One crack on its own rarely means subsidence. What matters is the pattern:

Diagonal cracks, wider than a few millimetres, especially if they're widening over time.
Cracks that are wider at the top than the bottom.
Cracks near doors and windows, or where an extension or bay meets the main wall.
The same crack visible both inside and outside.
Doors and windows that stick.
Wallpaper rippling at the joins.

Width alone doesn't diagnose subsidence: the pattern, the location, and whether the crack is changing matter just as much. A crack that appears suddenly is more of a concern than one that's sat there unchanged for years.

What causes it

In the UK, it usually comes down to what's under and around the house:

Clay soil. Clay swells when wet and shrinks when dry. Prolonged hot, dry weather can increase the risk on shrinkable clay, because the soil loses moisture and contracts. This is why it's most common on clay, like most of London and the South East. The trend is real: in the first half of 2026, following the warmest spring on record, the ABI (the insurers' body) recorded around £153 million in subsidence claims across roughly 9,000 households, with the average claim reaching a record £20,000.
Trees and large shrubs. On shrinkable clay, trees can draw significant moisture from the ground. But the risk depends on the species, size, distance from the building, soil and foundation depth, so a nearby tree isn't automatically a problem. (Environmental searches often flag larger trees within roughly 12 metres as worth noting, not as proof of harm.)
Leaking drains. Damaged underground drainage can change the moisture or stability of the ground around the foundations and contribute to movement.

How to read a “high subsidence risk” search

When you're buying, your conveyancer's environmental search may come back with a scary-sounding line: the ground has a “Very High” susceptibility to subsidence. Before you panic, understand what that means.

It's a susceptibility rating, not a diagnosis. It tells you the ground beneath or around the property has characteristics linked to a greater potential for movement. It does not mean the house is subsiding, or that it ever has. “Very High” clay susceptibility covers a huge slice of London and the South East, and the overwhelming majority of those homes are sitting there perfectly fine.

What it does do is tell you it's worth asking the house-specific questions below. Think of it as “check this properly,” not “walk away.”

The right first steps

If there's a high area rating, a crack, or a note of movement, do these cheap things:

Ask the seller, in writing, through your conveyancer: has the property ever had a subsidence claim, or been underpinned? This matters especially if cracks have been “previously patch-repaired”, someone filled them at some point, and without a record you can't tell whether it settled years ago or is still moving.
Get an insurance quote on the address before you exchange. You're not using the quote to prove the house is sound, you're checking whether the risk turns into a practical insurance problem. If cover is hard to get, unusually expensive, or comes with a big subsidence excess, you want to know before you're committed.
If movement is flagged, pay for a structural engineer. You've already paid one expert to raise the question, pay one to look at it properly. They'll assess the likely cause and significance of the movement, and whether further investigation or monitoring is needed. Sometimes establishing whether movement is still active takes time. A few hundred pounds can turn a scary unknown into a much better-informed decision.

It's also worth asking the neighbours, people on the same street, on the same clay, will tell you whether the area has a real history of problems.

And a reassuring point: subsidence doesn't automatically mean underpinning. Depending on the cause, the fix might be drainage repairs, managing a tree, or a period of monitoring. Underpinning is one possible intervention, not the default response to every crack.

The insurance reality (often the real sting)

Repair costs are only half the story. For a buyer, insurance is often the bigger issue.

A previous subsidence claim can make cover more complicated. When you apply, you'll need to answer the insurer's questions accurately, including any about past subsidence or structural movement. Depending on the case, some insurers may decline cover, charge more, apply a larger excess (a subsidence excess is commonly around £1,000, though it can be higher), or ask for evidence that the problem has been resolved. A subsidence history can affect your insurance options long after the physical repair is done.

On mortgages: active subsidence can make a lender nervous or hold up a purchase. But a home where the cause was found and fixed isn't automatically unmortgageable, arguably, a properly stabilised house is a safer bet than one that was never investigated.

So if there's any history or a high risk rating, get that insurance quote before you exchange.

If you're buying: should you walk away?

Sometimes the right answer is to walk. There's no universal rule, so work through it:

Is it historic or active? Get the engineer's view first. Historic-and-stable is a very different decision from active-and-ongoing.
How much do you actually want this house? If you're already compromising elsewhere, subsidence is a heavy thing to add on top.
What would it really cost, in money, time and hassle? Get realistic estimates for the work the engineer thinks may actually be needed, and leave a contingency for the uncertainty. Major structural work can be expensive and disruptive, so don't base your decision on the best-case number alone.
Can the seller resolve it first? Ask them to have it investigated and put right under their buildings insurance before you exchange. It protects you, but expect delays.
Beware sunk cost. The money already spent on surveys and solicitors is gone whether you proceed or not. Don't let it push you into the wrong buy.

Plenty of buyers happily take on a long list of repairs but draw the line at subsidence, a completely reasonable place to draw it. Equally, in a desirable area a well-documented, resolved case at the right price can be a sound buy that others are too spooked to touch.

If it's your own home

Don't panic, and don't ignore it. Get it assessed by a structural engineer or your insurer, fix the cause rather than just the crack, then monitor. And remember, underpinning isn't the automatic answer.

The bottom line

A crack is not a verdict, movement is not the same as active subsidence, and a “high risk” search is about the ground, not proof of a problem. Each signal tells you something different, and the skill is knowing which is which:

Property data points to the background and the local picture.
The conveyancer's environmental search rates the ground's susceptibility.
A survey tells you what can actually be seen at the property.
An engineer can investigate suspicious movement.
An insurance quote tells you whether the risk creates a practical problem.

Property Looker gives you the starting picture for an address; flood risk, the local area, price history and comparable homes, so you know what's worth a closer look before you offer. It won't tell you whether a house is subsiding: that's what the environmental search, the survey, an engineer and a ten-minute insurance quote are for. Our data tells you where to start. Those checks tell you what's really going on.