Guide
Subsidence risk by postcode: what actually determines it
Updated
People search for a subsidence postcode checker because they want a yes or no. Area data can tell you what the ground is like; it cannot tell you what your building is doing.
What drives risk in subsidence areas, postcode by postcode
- Soil type. Shrinkable clay soils change volume with moisture content, which is the mechanism behind the great majority of domestic subsidence claims in the UK. Sands, gravels and rock behave quite differently.
- Vegetation. Large, thirsty trees and hedges close to a building on clay draw moisture out of the soil. On non-shrinkable ground the same tree is far less of an issue.
- Drainage condition. Older drainage running close to foundations, particularly where it leaks, can wash out or soften supporting ground regardless of soil type.
- Historic mining and made ground. Former coal, chalk, clay or salt workings, and land made up with imported fill, create localised risks that have nothing to do with clay shrinkage.
- Foundation depth and age. Shallow Victorian foundations on clay respond to seasonal moisture change far more than modern foundations designed for the same ground.
Ground stability by postcode: where the data actually comes from
The British Geological Survey researches and maps shallow ground hazards including ground movement and publishes that work for professional use (BGS shallow geohazards). Commercial ground-stability and environmental search reports, of the kind a conveyancer routinely orders during a purchase, are built on datasets of that type, and insurers use their own underwriting models on top. There is no free official government postcode lookup that will tell a homeowner whether their house is at risk of subsidence, and any subsidence risk postcode checker, free or paid, that gives a confident verdict from a postcode alone is estimating, not measuring.
What a postcode cannot tell you
Two houses on the same street can sit on different made ground, have different foundation depths, different drainage condition and a mature oak on one side of the boundary only. In a subsidence check, postcode data sets the background probability. Whether your building is moving is established by looking at your building: crack pattern, whether the movement is progressive, and what the ground under that particular foundation is doing. That is the job of the inspection and monitoring sequence described in investigation and monitoring.
If you are buying
- Read the environmental and ground stability search your conveyancer obtains, and ask them to explain any flagged ground stability entry rather than skimming past it.
- If the survey mentions movement, get a structural engineer's opinion specific to the property before exchange. See report costs.
- Ask the seller whether there has been a previous subsidence claim and whether any monitoring, drainage repair or underpinning was carried out, along with the paperwork.
- Check insurability early with your own broker or insurer. A history of movement affects cover terms, and that is a conversation to have before you are committed, not after.
We do not sell risk reports and we do not hold postcode risk data. This page explains what determines risk so you can judge what a paid report is actually telling you. Nothing here is insurance advice.