The seismic retrofitting of frame structures using hysteretic dampers is a very effective strategy to mitigate earthquake-induced risks. However, its application in current practice is rather limited since simple and efficient design methods are still lacking, and the time-history analysis is more accurate but also time-consuming and computationally demanding. This paper presents the seismic retrofit of a reinforced concrete school building using metallic yielding dampers. At first, a fully multimodal procedure based on an adaptive version of the capacity spectrum method is applied. Then, the properties of an idealized bilinear model are defined using the seismic characteristics of the compound system thus accounting for the frame-damped brace interaction. Finally, an iterative procedure is developed to provide an optimal distribution of dampers. The proposed method overcomes the limitations of the design procedures in the literature that generally neglect the frame-damped braces interactions. Moreover, it addresses the main issues of seismic design of damped braces: effect of force demands applied to the frame due to the damper yielding and strain hardening, higher modes contribution, effect of soft-story irregularities, and torsion effect in asymmetric buildings. The effectiveness of the seismic retrofit is finally demonstrated by non-linear time-history analyses using a set of earthquake ground motions with various hazard levels.