Modal response spectrum may give unconservative results for highly torsionally stiff in-plan asymmetric buildings such as those composed of shear walls. Previous studies carried out by the authors suggest that the estimation of seismic response may be improved by means of the so-called additional mass eccentricity artifice. This consists of performing modal response spectrum analysis of a modified elastic model where the centre of mass is displaced a distance termed additional mass eccentricity. This paper presents equations for the calculation of additional mass eccentricities that may be used for assessment and design of in-plan irregular shear wall buildings located at soft soil sites of Mexico City.