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Factsheet: Natural ground subsidence

What is natural ground subsidence?

Natural ground subsidence is defined as the upward, lateral or downward movement of the ground that can be caused by a range of natural geological hazards and can affect the structural integrity/stability of a property and buildings. Whilst it is usually a natural process, the removal/presence of large trees or shrubs and building work may also be a factor. 

Certain types of geology have a higher susceptibility to movement. For example properties built on clay or chalk. The biggest cause of insurance claims is from shrink-swell clays. 

The first sign of subsidence is usually the appearances of cracks in walls, either in the internal plasterwork or external brickwork. The cracks usually appear suddenly, especially after long periods of dry weather, and tend to be:

  • Diagonal, and wider at the top than at the bottom;
  • Thicker than a 10p coin;
  • Found around doors and windows. 

Further information is available here: https://www.abi.org.uk/products-and-issues/choosing-the-right-insurance/home-insurance/subsidence/

What is the data based on?

The natural ground subsidence information used in Groundsure reports is derived from the British Geological Survey’s (BGS) GeoSure database. GeoSure provides an indication of potential near-surface ground instability related to particular natural geological hazards. There are six hazards (shrink-swell, compressible ground, collapsible ground, landslides, running sand and dissolution features). Each hazard is rated on a scale from A to E, where A and B are considered Negligible to Very Low and C to E are Low to High. Further information on these ratings can be found in our User Guide. 

GeoSure has been developed at 1:50,000 scale resolution, meaning it is accurate to 50m. This is due to the scale of the underlying geology. As a result, the highest risk within 50m is presented within the report. 

The ratings are based on the characteristics of the underlying geology, using the BGS 1:50,000 scale geological maps. The data is designed to indicate the potential for the ground to behave in a way that could cause subsidence or other ground instability issues. 

The data does not take into account any characteristics of a property. Many buildings, including more modern ones, are built to such a standard that they can remain unaffected in areas at elevated risks. 

The ratings are not an indication that subsidence has occurred at the site or that it will necessarily occur at the site. Such an assessment can only be made by an inspection of the area/property by a qualified professional.

What should a purchaser do next?

We always recommend obtaining a structural survey from a suitably qualified and experienced person. A list of local surveyors is available from https://www.ricsfirms.com

As this is involves someone attending the property, the results of this survey will supersede the Groundsure report. You may also wish to check whether the property benefits from an NHBC guarantee or other building warranty that covers structural issues. 

Professional advice should also be sought before altering the ground in any way, including planting or removing large shrubs and trees.