Existing steel structures are characterised by a wide variability in terms of structural conception and adopted constructional details. Moreover, older steel buildings are often irregular in plan and in elevation. These structures are usually non-conforming with normative provisions currently in force. Such unconformity can possibly lead to severe structural shortages. An existing six-storey steel building located in Naples is investigated as a case study. The selected building features both concentrically braced frames (CBF) along the transversal direction and moment-resisting frames (MRF) in the longitudinal direction, in which non-conforming beam-to-column joints were adopted. Multiple on-site surveys carried out by the Authors allowed the complete characterization of the structure. The global behaviour of the investigated structure was inspected by means of finite element simulations. Non-linear analyses showed that the case study has a poor seismic performance in both directions. Hence, a seismic strengthening intervention was designed and numerically checked. The efficiency of the proposed solution is presented by comparing the global response in both ante- and post-opera configurations.