Ask for the actual documents
In England, Building Regulations guidance (Approved Document C) says new buildings, extensions and conversions in areas where radon may be elevated may need to include precautions against radon, and points to BRE report BR 211 for the protective measures. Wales, Scotland and Northern Ireland have their own building regulations. Request details of any radon protection included in construction: a general statement about building quality does not tell you the measured radon level indoors. Keep drawings and product information with the property record.
Understand what was installed
Ask the developer to explain any membrane, provision for a sump or ventilation equipment. Establish whether a system is already active or merely prepared for possible future use.
Use measurements to verify performance
Preventive construction measures and an indoor radon test answer different questions. Ask a suitable professional whether testing is recommended once the home is occupied normally and follow the measurement provider’s instructions.
Plan for future alterations
Changes to floors, extensions and ventilation can affect the building. Keep the radon information available for future designers and contractors, and ask whether another measurement is appropriate after significant alterations.
Sources & further reading
GOV.UK: Approved Document C, including radon in new buildings (England) ↗UKHSA: official UK radon maps ↗UKHSA: measuring radon in homes ↗General guidance for property owners. Check the original source and obtain advice specific to your building.