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The problem associated with the risk assessment of radiation caused by the accumulation of radioactive radon gas in closed premises has not been solved to date. Additionally, there is no consensus regarding the set of parameters that should be used for radon hazard assessments of buildings. This paper presents the results of measurements related to radon assessments in the Russian Federation (radon flux density; RFD) and the Czech Republic (volumetric activity of radon in the soil air measured at a depth of 0.8 m and soil gas permeability). In the Russian Federation, RFD was measured in the city of Tomsk on the surface of loess loam using accumulation chambers and carbon adsorbers. In the Czech Republic, the volumetric activity of radon in the soil air and gas permeability were measured by the complexes RM-2 and RADON JOK, respectively, at a testing site located approximately 180 km southwest of Prague and characterised by brown forest and mountain sub-ash soils (cambisols) of the Bohemian Massif. The observed high variability of gas permeability in surface rocks and the radon flux density measured on the ground surface casts doubt on the reliability of assessments of radon hazard risks in the built-up areas. Based on the analysis of the results, other approaches are proposed for conducting radon hazard assessments of built-up areas.