The seismic assessment of historical unreinforced masonry buildings is a challenging task, because of their vulnerability to earthquakes and the uncertainties about the material properties. This work studies the case of the Kapela Sveti Nikola, a chapel in Petrinja (Croatia) which was damaged by an earthquake in 2020. The present report includes the seismic assessment of the church thanks to numerical simulation and hand calculations, and propositions for the retrofitting of the structure. The numerical simulation consists of static pushover analysis on a finite element model of the church performed on the software Abaqus. The approach for the modelling is the homogenised material model with 2D shell elements. Local failure mechanisms are also analysed through hand calculations. This seismic evaluation of the church, along with the observed damage on the church’s structure, is the basis for the propositions of strengthening measures. Some retrofitting ideas are advanced and their adequacy in the case of this church is discussed, but no structural analysis of the retrofitted building is carried out.