The addition of a green roof to existing multistorey buildings in urban areas is a particularly interesting environmental upgrading solution, promoting sustainability, but it changes their seismic performance. In this study, the influence of adding an extensive green roof to a six storey office building, with steel structure, is assessed using a pushover procedure. The capacity curves required by this procedure were computed with the structural analysis software EvalS. The choice for an extensive green roof, instead of an intensive one, is justified, since i) it is more likely to be adopted in existing buildings and ii) it may avoid having to strengthen of the structure. The pushover procedure – seismic nonlinear static analysis – is one of the possibilities proposed by the Eurocodes to evaluate the seismic performance of buildings.