A hybrid procedure for seismic retrofit of an existing R.C. structure by using base isolation system is discussed in this paper. The case study is a high school building in Naples, built at the end of 1970s, as result of a gravity-load design approach. It consists of three jointed R.C. portions, having different features in plan and elevation. In particular, “Building 1” and “Building 2” are characterized by resisting frames only in transversal direction. These ones support the actions transferred by the floors, whose load-bearing elements are warped longitudinally. Both the two buildings are characterized by some R.C. shear walls, in correspondence of (internal/external) staircases. The three portions are separated by tight joints, whose width is inappropriate to avoid pounding damages due to earthquake shaking. Considering that the structure has no resisting frames in longitudinal direction, this school building has low seismic capacity longitudinally and a high lateral deformability, with a fundamental vibration period of TAS-IS =1.37s. In this case, seismic retrofit by using base isolation system has been necessary to improve its seismic response. On the other hand, the main aspects of modern capacity design approach are not complied with between dissipative and no-dissipative components. So, it is assumed that the current structural system has no ductility reserves, on local and global scale. For the seismic retrofit of the existing structure an “all rubber” base isolation system (BIS) has been assumed by using ϕ600 HDRBs, allowing to reach a TISO = 4.0s. To maximize the effectiveness of BIS, the three bodies have been connected at each floor level and the upper structure has been strengthened previously, reducing to TFB=0.30s the fundamental period of the strengthened fixed base solution. The proposed hybrid solution for seismic retrofitting merges two conceptually opposite design strategies, opting for BIS applied to an existing structure, stiffened ad hoc by using shear walls.