A map is about probability
Official mapping identifies areas where elevated indoor radon is more likely. It draws on geology and measurement information. The category for an area is not a direct test result for every building inside it.
Check the actual address
General county lists and postcode summaries can hide local differences. Use the official UKradon information and address search for the property you are considering, and keep a copy of the report.
Do not overlook below-ground space
Basements and underground spaces need separate consideration. A low mapped probability should not be used to rule out measurement where the use and construction warrant it.
Turn information into a testing decision
Follow the testing advice in the official report. If a home is tested, use the laboratory’s result to decide whether mitigation is needed. A neighbouring property’s result does not establish yours.
Sources & further reading
UKHSA: official UK radon maps ↗General guidance for property owners. Check the original source and obtain advice specific to your building.