This paper proposes a displacement based method for the design of an energy dissipating system in order to upgrade the seismic behaviour of existing masonry structures. The dissipating system is composed of hysteretic dampers (ADAS) and external concrete walls. The devices are installed within the concrete walls at the same height of the structure floors and then connected to the structures by steel connectors. In the first part of the work a simplified model of the overall structure-dissipating system is developed. The results obtained from inelastic time-history analyses for the simplified model are compared to those of a more refined model. The agreement between the two models allows the adoption of the simplified model also for design purposes. In the second part of the work a displacement based damper design procedure is developed. This procedure is based on the knowledge of the nonlinear static response of the existing structure. The design procedure is tested by means of inelastic time-history analyses considering four masonry test structures with different types of hysteretic behaviour. The analyses are carried out considering two groups of spectrum compatible accelerograms. The results of the analyses are then compared with the seismic behaviour expected from design.