The use of exoskeletons, intended as additive structures applied in a widespread way externally to the building to be protected, constitutes a valid alternative to common intervention techniques for the seismic retrofit of existing buildings. Among many typological solutions present in the literature, in the present work, the orthogonal 2D steel exoskeletons (called EXO 2D_λ) have been analysed in detail. These systems, made as truss shear walls placed orthogonally to the facade of the building, can be effectively used for the strengthening of low-mid-rise buildings. In the present work, after a thorough state-of-the-art and a critical analysis of recent realizations, a kit-constructive system, suitable for prefabrication, was conceived. [ABSTRACT FROM AUTHOR]