Ninety percent of the building in Nepal practice open stories on the ground floor for commercial shutters/ parking and close the upper floors with infill masonry for residential purposes. During earthquakes, these buildings collapse on the open floor due to a sudden reduction in stiffness on that floor. Various retrofitting techniques like steel jacketing, carbon fiber wrapping, and base isolation, are implied to improve the performance of these buildings however these methods are not economically feasible. On the other hand, the solutions suggested by many building codes or indices do not give a satisfactory level of performance. In this paper, a mechanistic model, which can identify the extent of the damage of an existing soft story building is defined as an index defined by authors as a soft story index (SSI). Static pushover analysis (SPO) was used to compare the performance of a soft story frame i.e. (subject structure) to that of a uniformly infilled RC frame i.e. (baseline structure). As the baseline structure is uniformly infilled on all the floors, it is considered an ideal frame that does not have a soft story phenomenon. A ratio between the fundamental frequency of the subject structure and the baseline structure is defined in this paper to be the soft story index (SSI). The SSI value ranges from 0 to 1 with the value 1 being the ideal case to avoid a soft story and with the reduction in the value of SSI the structure would have a higher chance of having the soft story phenomenon. Furthermore, in this paper, the SSI index is used to improve the performance of a typical frame through retrofitting and the dynamic behavior of the structure is analyzed using incremental dynamic analysis (SSI) to verify the developed index is applicable to improve the soft story problem in a structure.