What the indicative map tells you
Radon comes from natural materials in the ground. Geology affects how readily it can move into a building, so the potential varies by area. The open BGS/UKHSA Indicative Atlas gives each 1 km grid square the highest radon-potential class found within it.
The bands—less than 1%, 1–3%, 3–5%, 5–10%, 10–30%, and 30% or more—describe the estimated percentage of homes in the mapped class above the UK radon Action Level. They are not the chance that this particular home has a high measurement.
What is a Radon Affected Area?
UKHSA calls an area radon affected when the estimated proportion of homes at or above the Action Level is 1% or more. That threshold is a prompt to check, not a finding that an individual house has a problem. Building design, ventilation and ground conditions at the address all matter.
When to investigate further
If the indicative result is 1% or above, ask your conveyancer whether a definitive address-level radon check is appropriate. If you need to know the level inside the home, only a measurement can answer that. A map cannot replace a test, and SashMark does not replace a formal radon search.
It is useful to raise this before buying so you can understand any previous tests or remedial work and decide whether you need more evidence. Even a low mapped band is not a guarantee about one building.
See radon in context
SashMark can show indicative radon potential alongside non-coal mining, historic landfill, coal-risk and flood checks in a property report. It uses the best trusted location available, but a postcode-centre fallback and a 1 km grid must not be mistaken for a property-specific assessment. For the wider decision, see how to check a property before buying.