The present study evaluates how the soft first story irregularity condition should be defined: (a) as a significant reduction of the lateral shear stiffness of all resisting frames within a given story, as established in the seismic provisions of Mexican building codes or, (b) as a substantial reduction of the lateral shear stiffness of one or more resisting frames within a given story, as proposed by the author. Both definitions are evaluated through nonlinear dynamic analyses of buildings systems with a suspected soft first story condition in order to discern which option is closer to define the soft fist story condition.