•Applicability of the nonlinear static procedure when applied to existing URM buildings.•Critical analysis and comparison of nonlinear static procedures proposed in international Standards.•Seismic assessment in presence of structural irregularity and not rigid diaphragms.•Criteria to properly define damage levels associated to the global response of URM buildings.•Refinements for the target displacement evaluation of URM buildings. The nonlinear static procedure (NSP) is commonly adopted by practitioners for the seismic assessment of unreinforced masonry (URM) buildings, although a systematic validation of the procedure when applied to this type of buildings still lacks in the literature. The aim of this paper is to contribute to the validation and the improvement of the NSP when used for the seismic assessment of irregular URM buildings. The effectiveness of the NSP is tested on numerical models representative of URM buildings that present both in plan and in elevation irregularities. The concept of irregularity is also extended to that consequent to the presence of flexible/stiff diaphragms, so frequent in existing URM buildings. To this aim a total of 13 numerical models representative of 3 and 4 stories URM buildings were developed. The results derived by the application of the NSP are compared with the nonlinear dynamic procedure, considered as the reference solution. Based on the findings of the present research, original contributions are provided to: (1) the load patterns to apply for the pushover analysis; (2) the identification of damage levels; (3) the evaluation of the target displacement and the corresponding intensity measure that causes its attainment.