Displacement-based design methods (DBDM) have been extensively studied and developed to become applicable design tools for different types of structures. For ultimate limit states where non-linear behaviour is expected, most of these methods assume that the nonlinear maximum response of a structure can be approximated through a single-degree-of-freedom (SDOF) reference system associated with its fundamental mode. For this reason, most of these methods have been studied extensively in planar frames and buildings with moderate torsional effects. This paper presents a modification of the DBDM proposed by López and Ayala [1] to be applied in the design of 3D in-plan asymmetric buildings by means of spectral decomposition. This approach would allow an improvement in design of buildings where torsional effects are significant.