The retrofitting of existing reinforced concrete structures is of paramount importance due to aging infrastructure, changes in design codes, and the need to enhance resilience against seismic events. This paper addresses this critical issue by presenting the results of an experimental campaign focused on evaluating the shear performance of old-to-new concrete interface, considering the application of a thin (5 cm) concrete overlay. Both monotonic and cyclic shear tests were conducted to assess the behavior under static and seismic loading conditions. Mechanical anchors were installed into the existing concrete as connectors. The study investigated the influence of different concrete types, including normal-weight and lightweight concrete, used for the overlay. The experimentally obtained shear strength values were compared with predictions derived from several established design codes and guidelines: Eurocode 2, Model Code, ACI 318-19, and EOTA TR066. The analysis highlights the key differences in the formulations provided by these codes and discusses the parameters that most significantly affect the predicted and experimental shear resistance at the interface.