Subsidence surveys: warning signs, causes and what to do

Subsidence is a term that can cause major worry for homeowners and buyers alike. It can be costly to remedy and raises serious questions about a property's structural integrity and mortgageability. For an overview of all issues a survey can flag, see our common survey problems guide.

Here, Surveyor Local looks at what subsidence is, how it is identified during a subsidence survey, how much any remedial work might cost and what it means for your purchase if the surveyor has flagged it as a concern.

• Subsidence occurs when the ground beneath a property sinks, putting pressure on the building's base and causing structural damage.
• It is different from settlement, which is normal and expected, and from heave, where soil expands and pushes the structure upwards.
• Key warning signs include diagonal cracks wider than 3mm, sticking doors and windows, sloping floors and cracks visible on both interior and exterior walls.
• A Level 2 survey will flag possible subsidence and recommend further investigation. A Level 3 survey provides a more detailed structural assessment.
• If subsidence is suspected or the property has a history of it, a Level 3 survey followed by a specialist structural report is strongly recommended.
• Historic, remedied subsidence is generally mortgageable with the right documentation. Active subsidence will deter most lenders.

Subsidence describes the downward movement of the ground's surface. In the context of property, it refers to the situation where the ground beneath a building sinks, pulling down on the foundations. This often causes structural damage as the base becomes unstable, while the upper parts of the building are placed under increasing pressure.

The most common cause is clay soil shrinking around a building's base during dry periods, creating pockets into which the weight of the structure is forced downward.

Settlement is different. It is an expected and gradual process, particularly in new-builds, as the structure beds into its foundations and finds its natural position. Settlement is slight and rarely leads to the level of damage associated with subsidence.

Heave is essentially the opposite. Here, the earth around the base of a property expands and forces the structure upward. This is often triggered by excessive wet periods or unmanaged flooding, where the ground takes on moisture and swells. Heave exhibits similar symptoms to subsidence and is something your chartered surveyor will assess as part of the report.

There are various telltale signs of subsidence that a surveyor will examine carefully during an inspection.

Diagonal cracks

These are the most obvious symptom, particularly where the crack is wider at the top than at the bottom, indicating that the upper part of the building is shifting away more than the lower sections. Subsidence cracks wider than 3mm — roughly the depth of a 10-pence coin — will be given further analysis.

Cracks at points of weakness

In addition to diagonal cracks, surveyors look for cracks at the natural weak points of a structure, typically the corners of door and window frames. Where a property has an attached extension, this junction is also a common source of subsidence-related cracking.

Cracks visible on both sides of a wall

Where a crack is visible on the interior and can also be seen in exactly the same location on the exterior wall, this is a strong indication that the shifting is structural rather than superficial.

Sticking doors and windows

Where doors or windows that once opened freely now stick or cannot be operated properly, this can indicate that the frame has warped under the pressure of structural instability.

Wallpaper and decorative issues

Rippling or torn wallpaper, displaced cornicing or cracked decorative finishes can all be indicators of structural change within the building. These are subtler signs but form part of the overall picture built up during a subsidence survey.

Uneven or sloping floors

Floors that have become uneven or slope noticeably, particularly toward an external corner, are another common symptom. Gaps forming between the floor and skirting boards or door frames may also be present.

Exterior signs

Surveyors will look for external evidence including dips, cracks and lifting in hardstanding surfaces such as driveways and paths. These can indicate that a void has opened up beneath the surface, potentially linked to subsidence damage affecting the building's base.

Signs of heave

Where heave is the cause, the symptoms can appear similar to subsidence, but the direction of movement is upward rather than downward. Surveyors will look for floors that have lifted, doors that now catch at the top of the frame and cracks that are wider at the bottom than the top. Heave is particularly associated with properties where nearby trees have been removed, causing previously dry earth to rehydrate and swell.

Clay soil is prevalent across much of the UK and represents a significant subsidence risk in many cases. As clay dries out during dry spells, it shrinks and can pull away from a building's foundations, creating the conditions for downward movement.

Significant planting close to a property can also contribute, as root systems draw moisture from the earth around the foundations. This is why guidance exists on managing what is planted near buildings.

A leaking or failing drain is another common trigger. Water escaping underground carries soil particles away from the building's base over time, gradually opening voids beneath the structure. This is a particularly insidious problem because it can be slow and difficult to detect without investigation.

Mining activity in certain parts of the UK can also lead to ground instability. Where a property sits in a known mining area, this will be noted as a subsidence risk factor.

Depending on the severity of the signs observed, newer or apparently stable cracks may be monitored over a period of time before further action is taken. Where the situation appears more serious or is actively worsening, a specialist structural assessment will be recommended to establish the extent of the problem and advise on appropriate remedial action.

The cost of remedying subsidence depends on the severity of the problem, how many walls are affected, the ease of access for any machinery required and the size of the property.

Structural engineer's report: appointing an engineer to assess the subsidence typically costs between £200 and £1,200, occasionally more for larger or more complex properties. This report is usually a prerequisite before any remedial work begins.

Underpinning: this involves strengthening the foundations by excavating deeper or wider and filling the void with concrete, so that the new footing rests on more stable ground. The cost is around £1,000 to £1,750 per metre, which can amount to £10,000 or considerably more depending on the extent of the problem and the size of the property.

Resin injection: a faster and less disruptive alternative to underpinning. A liquid resin compound is injected into the weakened earth beneath the building, where it expands and solidifies to fill the void. It can also help to re-level walls that have dropped. Costs are typically around £7,000 to £8,000 for an average home, again dependent on size and severity.

As with all remedial work, always obtain a minimum of three quotes from reputable specialists before committing to any course of action.

The answer varies case by case. The key question for lenders is whether the subsidence is historic and remedied, or currently active.

Historic subsidence: where subsidence has been professionally addressed, lenders will typically require an engineer's report confirming the current situation, a certificate of structural adequacy and evidence of current buildings insurance. With these in place, many lenders will proceed.

Active subsidence: where subsidence is ongoing and unremedied, most lenders will decline to offer a mortgage until the problem is resolved. Properties in this condition do sometimes come to market at auction, where buyers typically proceed without mortgage finance.

Buildings insurers take a similar view. Obtaining insurance on a property with active, unremedied subsidence is difficult, and even properties with a subsidence history can attract higher premiums or require specialist cover. Specialist insurers do exist for these cases, and a broker with experience in non-standard cover will be best placed to advise. It is worth consulting one early if you are concerned about the insurability of a property you are considering buying.

A Level 2 survey will flag possible subsidence and note the signs observed, but will not go into significant structural detail and will typically recommend further investigation rather than providing a full assessment.

A Level 3 survey is the appropriate choice for any property where signs of movement are present, where the property is older or where there is a known history of subsidence. The survey will analyse the cracks and other symptoms in detail, assess the overall structural integrity and make recommendations on next steps, including an indication of the likely costs and risks involved.

A subsidence survey of this depth is often only the first step. An engineer's report will also be needed to fully understand the extent of the problem and to satisfy the requirements of a mortgage lender.

Not sure which survey is right for your property?  Compare RICS surveys.

A RICS Level 2 or Level 3 survey is the right first step for any property purchase. Get a free quote or call our friendly team on .