Updated legal situation
The latest radiation protection basic standards directive of the European Commission was published in 2013 and is entitled "Directive 2013/59/Euratom laying down basic safety standards for protection against the dangers arising from exposure to ionizing radiation". It contains detailed and far-reaching health protection measures regarding radon. These were implemented in Austrian law with the Radiation Protection Act 2020, the General Radiation Protection Ordinance 2020 and the Radon Protection Ordinance.
Further information on the Radiation Protection Act 2020 can be found here, and further information on the Radon Protection Ordinance here. An overview of the legal regulations and guidelines in radiation protection can be found here.