This part of ISO 11665 specifies requirements for the determination of the activity concentration of radon in all types of buildings. The buildings can be single family houses, public buildings, industrial buildings, underground buildings, etc. This part of ISO 11665 describes the measurement methods used to assess, during the initial investigation phase, the average annual activity concentration of radon in buildings. It also deals with investigations needed to identify the source, entry routes and transfer pathways of the radon in the building (additional investigations).Finally, this part of ISO 11665 outlines the applicable requirements for the immediate post-mitigation testing of the implemented mitigation techniques, monitoring of their effectiveness and testing of the sustainability of the building’s behaviour towards radon.This part of ISO 11665 does not address the technical building diagnostic or the prescription of mitigation work.