As result of a benchmarking process on foreign computer codes for design of masonry buildings, the guidelines for the development of a Portuguese design software for plain masonry buildings are presented. The related software was inspired on Italian methods, particularly the RAN, and allows to perform a global response analysis of buildings, based on the assumption of a cumulative response of storeys and walls. Besides the assumptions of behaviour and equilibrium, modelling of stiffness and collapse mechanisms, the main features of the formulation adopted are presented. As a form of validation a case study is presented. © Civil-Comp Press, 2009.