In this thesis, the seismic performance assessment of the former administration building of RŽS Idrija is investigated. The first part of the thesis presents the behaviour of masonry buildings under seismic actions, and typical failure mechanisms of masonry walls, followed by the provisions of the Eurocode 8-3 standard, which deal with seismic assessment of existing buildings. Further on, the simplified N2 method based on non-linear static (pushover) analysis is introduced, followed by the presentation of 3Muri software. The second part of the thesis begins with a description of the administration building, which was build out of stone and brick masonry walls without adequate floor-to-wall connections. Not knowing the exact quality of the existing materials, three sets of mechanical properties for both types of masonry were obtained from the literature. The seismic performance assessment was based on five models of the structure each with different combination of mechanical properties of stone and brick masonry. The results of the deterministic analysis have shown that the building is incapable of withstanding the expected design ground acceleration ag = 0,23 g and does not meet the requirements of the Eurocode 8 standard. The great vulnerability of the structure is a consequence of very low shear strength values of the masonry, which were assessed from previous studies and, in accordance with Eurocode 8-3, divided by the confidence factor CF = 1,35. Different combinations of mechanical properties of masonry have lead to different types of failure mechanisms of the structure. Due to small differences between shear strength values of the materials the seismic resistance of our models did not differ significantly. The seismic shear coefficient SRCu was between 0,09 and 0,12 which complied with the coefficient estimated in the POTROG (2013) project. It is estimated that the limit-state peak ground acceleration is most likely underestimated, which is the consequence of safety factors that need to be considered in the case of incomplete knowledge of the structure.