When performing the seismic analysis of existing unreinforced masonry buildings, flexible diaphragms are often present. In case the flexibility of these elements is particularly significant, the execution of meaningful pushover analyses with fixed load pattern, performed on a three-dimensional model of the structure, is widely recognized to be problematic and tends to provide information only on the failure condition of the weakest wall. Thus, single bidimensional models of each structural wall are usually required and separated pushover analyses are carried out on each of them. In this paper, different modelling strategies to model single walls and entire buildings to perform nonlinear seismic analysis, are compared. In particular, simplified approaches included in current building codes are compared with more refined approaches, to find out the main differences among code-compliant and advanced models and highlight possible weaknesses of code-compliant strategies.