Subsidence & Radon Home Risk Assessor

The tool maps your postcode to BGS regional shrink-swell overlays, the UKHSA radon atlas band for the area, and Coal Authority legacy-coalfield zones.

Bungalows (shallow foundations) carry the highest clay-subsidence susceptibility; flats the lowest. The tool applies a property-type multiplier to the raw ground-risk score.

Pre-1850 properties typically have rubble or stone foundations that pre-date modern building codes — the tool flags them for a RICS Level 3 survey.

Oak, willow, poplar and sycamore within ~10m of a clay-built property are the biggest subsidence-claim trigger. Basements materially increase both radon exposure and landslip sensitivity in hilly areas.

The tool multiplies your current premium by a subsidence-risk multiplier (1.0–1.6×) and an additional 1.25× if the area is a coal-legacy zone. This gives a realistic annual uplift.

Red "danger" alerts require action before exchange (radon testing for band 5+, CON29M for coal zones, arboricultural survey for high clay + trees combinations). Amber warnings are strongly advised. The 10-year outlook combines insurance uplift + amortised mitigation + expected claim exposure.

The BGS GeoSure dataset rates every 1km² of Great Britain on six ground-hazard dimensions: shrink-swell, running sand, compressible ground, soluble rocks, collapsible deposits and landslip. Each is scored from A (none) to E (very high). London Clay, Gault Clay and Oxford Clay produce most of the E-rated shrink-swell areas — covering roughly the home counties, Oxford, Cambridge, Essex, Lincolnshire and parts of Somerset. The full GeoSure dataset is licensed to conveyancing search providers; the public summary is what this tool uses.

Radon gas seeps from granite and limestone bedrock through cracks in suspended timber floors, service-entry points and solid floor construction joints. Living rooms and bedrooms on the ground floor are the highest-exposure spaces. Mitigation usually involves a radon sump (a small excavation with a fan that draws air from under the slab and vents it outside) costing £2,000–£3,500 installed, or positive-pressure ventilation (pushing filtered outside air into the property to dilute radon) at £1,200–£1,800. Both reduce indoor radon by 75–95%.

Since 2017 the Coal Authority’s data has been centralised and digitised, making CON29M searches faster and more complete. The search now includes not just deep and shallow coal workings but also subsidence claims on the property or neighbours, mine-gas venting, future workings in planning, and gas-emission monitoring points. Purchasing in a coalfield area without a CON29M is one of the most common causes of post-completion disputes — conveyancers who skip it may be negligent.

The UK subsidence insurance market is dominated by four insurers (Aviva, Direct Line, LV= and RSA) for standard risks, plus specialist panels (Subsidence Claims Bureau, Pen Underwriting, a dozen Lloyd’s syndicates) for high-risk properties. Key rules: (1) Prior claim history attached to the PROPERTY, not the owner — a subsidence claim 20 years ago still raises premiums today. (2) Excess typically £1,000 on standard policies but £2,500–£10,000 on specialist cover. (3) "Heave" (upward soil movement after tree removal) is covered under subsidence sections of most policies but check explicitly.

Commission professional advice if you see any of: (1) existing Category 3 or 4 cracking (wider than 5mm or continuous through walls and render), (2) the property has an active or recent subsidence claim and no monitoring period completed, (3) the plot is under 10m from a tree >15m tall on clay soil with no arboricultural survey, (4) the plot is within 50m of a mapped deep coal working with gas-emission monitoring, (5) a surveyor or BGS GeoReport flags running-sand or compressible-ground hazards and no ground investigation has been done. These are scenarios where the 10-year cost can easily exceed £30,000 and mortgage lenders may retain funds or refuse to lend.