Radon gas is produced from the disintegration of uranium found in various types of soils and rocks. The World Health Organization (WHO) states that inhalation of radon gas is the second leading cause of lung cancer after smoking. European and Spanish legislation establishes an average annual threshold of 300 Bq/m3 for dwellings, public buildings, and workplaces. To implement effective mitigation strategies, it is essential to identify the main sources of radon entry into buildings. In this study, a thorough investigation was conducted in a large ground-floor office building located in northwest Spain to identify the main geological determinants and sources of radon that may explain the variability of radon concentration throughout the building. The methodology followed involved characterizing (physical, mechanical, chemical, and mineralogical) various granite samples taken from the soil and applying Ground-Penetrating Radar (GPR) techniques across the building floor. The analysis of the rocks revealed two types of granite present in the building, and clear relationships were found between indoor radon levels and geological characteristics. Furthermore, the GPR images also showed different disintegration and weathering of the soil rocks, proving the existence of two types of granites.