The interaction of soil and structure significantly alters displacement response of the framed structures. This effect is mostly important for the buildings resting on soft soils. Including these effects in the dynamic analysis, the lateral displacements and the drift demand of the building increase depending on the type of the underlying soil. The variation in the drift demand due to the interaction effects also changes the performance level of the structure, as well. Since the interstory drift ratio is one of the most important demand parameter, it is essential to integrate the effects of soil structure interaction within the structural design process and the performance assessment. In this study, the drift demand of an existing multistory building is obtained using an improved technique for dynamic analysis of the soil-structure systems. The technique is based on the analysis of the multistory buildings under the seismic wave propagation employing Finite Element Methods (FE) for the superstructure and Boundary Element Methods (BE) for the underlying soil medium. The lateral displacement response at each story level is used to determine maximum interstory drift ratio of the building and to estimate structural performance level.