Environmental spread of radon gas (222Rn) has been intensively investigated in the last few years due to its harmful effects on human health. A concept of radon index is used to characterize the geogenic radon potential of the terrain, as the latter gives the probability of the presence of radon gas concentration in a building, the genesis of which is directly related to the influence of the earth's surface. One of the approaches for quantifying the radon index is based on a multivariate cross-tabulation, which includes two parameters – radon concentration in soil gas and gas permeability of the earth layer (at 80 cm). The geology of the specific site (approx. 350 m2) is characterized by the Pliocene clayey-sandy Formation, covered with Quaternary sediments. From tectonic point of view, the site as a part of Sofia Graben, has being subject of events related mainly with the Late Alpine deformations heading to very complex structures and fault systems. In situ measurements performed in the summer of 2023 at ten distinct points at the surface and 11 distinct points at two meters in depth from the surface vary as follows: from 44.2 to 189.0 kBq/m3 (radon soil gas), from 2.0E-11 to 1.8E-13 m2 (soil gas permeability), and from 0.10 to 0.17 µSv/h (gamma dose rate). Based on that, the radon index of the site is determined from “medium” to “high”, with a predominance of the latter. One of the reasons for the high radon potential is the presence of sub-faults connected with the major Sofia fault system and their influence on the radon concentration. Therefore, in future investigations of the radon index, a very detailed survey of the site's geology is needed in the territory with appearing fault systems. In addition, based on the determination of the radon index, the forthcoming construction activities in such areas will be advised to use preventive measures during the construction of the new building to avoid future radon gas influxes on the premises.