In this work the seismic resistance of a reinforced concrete building, before and after a seismic retrofitting, is evaluated. The building was built in the late seventies in Eastern Sicily, a high seismic area where regulations have been enforced only since 1981, therefore it was not designed to resist earthquake loading. Recently a retrofitting project for the considered building has been developed by a professional engineers team. The project includes both a partial reinforcement of the existing structure and a base isolation. In this part I the problem of the evaluation of the seismic resistance of the existing building is addressed by means of three-dimensional nonlinear analyses as well as of simplified procedures. The building has been modelled as a space-frame in which the nonlinear behaviour has been introduced by means of lumped plasticity elements. The performed three-dimensional nonlinear analyses have also been used for the definition of equivalent nonlinear single-degree-offreedom systems. Such simplified systems allow the estimation the level of peak ground acceleration (PGA) which the building can withstand according to a prescribed design spectral shape and therefore the evaluation of its seismic resistance with reference to specified seismicity classes and local site conditions. In the part II an advancement of the above-mentioned procedure, specialised for buildings retrofitted by means of traditional and innovative techniques, is described and applied to the case-study building.