What subsidence is, and how it differs from heave and settlement
The Financial Ombudsman Service defines subsidence as the ground beneath a building sinking and pulling the foundations down with it. It is usually caused by the ground losing moisture and shrinking.
Two related terms are often confused with it:
- Heave is the opposite movement: the ground rises when the soil expands, for example when clay takes in water again.
- Landslip is downward movement of sloping ground.
Cracks can also come from other causes. The Ombudsman is clear that cracks do not automatically mean subsidence. Settlement, poor workmanship and thermal movement can all cause them. That is why diagnosis needs an engineer, not a guess from a photograph.
Signs people notice
No single sign proves subsidence. Things people commonly notice, and which are worth reporting to a professional, include:
- New cracks, or existing cracks that get wider or longer over time.
- Doors and windows that start to stick or no longer close properly.
- Gaps opening up where an extension meets the main building.
Because other causes exist, the useful question is whether the cracks are moving. The Ombudsman expects insurers to monitor cracks to see whether the building is still moving and whether it is seasonal. It says monitoring over a year may be needed so that all the seasons are covered.
What causes it
The causes the Financial Ombudsman Service lists include:
- Clay shrinkage: soil with a high clay content drying out.
- Trees: roots absorbing water from the ground.
- Escape of water: leaking drains or pipes washing soil away or softening the ground.
- Solution features: erosion under a building, often in limestone or chalky soil, which can form a sinkhole.
- Mine collapse: including disused mines.
- Poor ground: badly prepared or decomposing fill material.
The British Geological Survey explains the clay mechanism: clay minerals take in water and swell when wet, then lose water and shrink as they dry. Moisture changes can come from weather, from vegetation including growing or removing trees, and from human activity. The movement mostly affects the top two metres of ground and can damage foundations, pipes and services.
This has a practical consequence for trees. Removing a large tree quickly can lead to heave as clay re-absorbs water, and a tree may be protected, so get advice before anything is felled.
Mining
If the area was mined for coal, there is a separate route. The Mining Remediation Authority handles claims for coal mining subsidence damage. GOV.UK says to have a chartered surveyor check whether the damage was caused by coal mining, send in the damage notice form, and expect an engineer to inspect. The GOV.UK guide to your rights says claims must be made within 6 years of the damage becoming apparent. That guide was last updated in January 2019, so confirm the current rules. Our guides cover who pays for mining subsidence.
What to do if you suspect subsidence
- Do not panic and do not start repairs. Photograph the cracks, date them and measure them, so movement can be tracked.
- Tell your buildings insurer. The Ombudsman warns that an insurer might cancel a policy if you knew about subsidence when you took it out and did not say.
- Expect investigation. The Ombudsman describes thorough investigations as typically involving shallow and deeper boreholes, samples and video inspection of nearby drains, with crack monitoring that may run for a year.
- If you are not insured, or the insurer declines, instruct a structural engineer who can tell you whether the movement is continuing and what is causing it.
- Keep every report, letter and invoice. A buyer or lender will ask for them later.
Cost: why we do not give a figure
Costs vary enormously. A leaking drain that is repaired is a different problem from clay shrinkage around a large tree, and different again from underpinning. The BGS estimates that shrinking and swelling ground has cost the economy about £3 billion over the past decade, but that is a national figure, not a guide to what one house costs.
We have not put a per-house figure on this page because we could not find a current figure from an official or insurer-association source. Ask an engineer for a written scope and quotes, and ask your insurer what it will cover.
Mortgage and insurance effects
Each lender sets its own rules. A valuer who sees signs of movement may ask for a structural engineer's report before the lender agrees to lend, and the lender may then decide whether to lend, to lend less, or to decline. Do not assume that a past problem rules a property out, or that it does not matter; ask the lender, through the solicitor, early.
For insurance, a property with a history of subsidence may have fewer insurers to choose from, and you must answer the insurer's questions honestly. Our guides on past subsidence and whether to buy and insuring a house on clay explain the usual questions.
Buying a house with subsidence risk: checks to make
- Ask about history. Has there ever been a subsidence claim? Was the insurer notified? Has monitoring been done, and what did it conclude?
- Ask for paperwork. Engineer's reports, monitoring results, repair specifications and any certificates or warranties for completed work.
- Choose a survey that fits. RICS recommends its Level 3 survey, the most comprehensive report, for large, older or run-down properties, unusual or altered buildings, or where major works are planned. Ask the surveyor whether they will comment on movement.
- Check the ground. Our free property risk checks and the Risk Report show ground stability from British Geological Survey GeoSure data. That data covers six natural ground hazards, including shrink-swell clay, and tells you about the potential for movement in the area, not whether a particular house is moving. Only a survey or engineer can say that.
- Check the trees. Mature trees close to the building on clay deserve a closer look.
- Check mining. In a coalfield, the mining report includes known mining information and claims history. GOV.UK's guide notes that claims are time-limited, so a new owner's claim could be rejected if a previous owner let the deadline pass.
Selling a house with subsidence
If you are selling, honesty and paperwork are your best tools.
- Answer the seller's property information form truthfully, including any subsidence or structural questions. Wrong answers can cause legal problems after the sale.
- Put together a file: the engineer's report, monitoring results, insurer correspondence and details of any repairs.
- Tell your solicitor early, so that the buyer's questions can be answered without delay.
- Think about who your likely buyer is. A mortgage buyer will depend on a lender's view, while a cash buyer will not need one.
We cannot tell you what subsidence does to a price or how long a sale will take. Those depend on the cause, the evidence and the buyer. If the problem is a wider structural one, see our page on selling a house with structural problems, which explains the options.