In this paper a church building characterised by a basilica plan is investigated for deriving indications on its vulnerability to seismic actions. Linear analysis of the fiill 3D model of the complex structural system is carried out through FEM in the static and dynamic case. The results of the analysis in terms of elastic demand on the different 2D elements identified in the 3D model and constituting the church are then compared to the ultimate capacity of the single 2D elements. The effect of a frequently used upgrading procedure consisting in the insertion of rigid diaphragms at different levels is also investigated. The results of the study, applicable to this specific case study, but somehow extendable to other basilica type churches characterised by similar global structural properties, confirm the susceptibility of these buildings to extensive damage and possibly to collapse, and therefore the need of upgrading interventions. The insertion of rigid diaphragms has not proved to be a surely efficient means for increasing the seismic capacity of the building.