In this article, performance evaluations of a 3D building strengthened by additional shear-walls are presented as a case study by considering the foundation effects. Soil-structure interaction is included in the nonlinear static analysis of the building and performance evaluations are made according to Chs. 6 and 8 of FEMA440. Different foundation types such as mat foundation, continuous or single footings are considered in the analysis. Tensionless elastic-plastic Winkler soil model is used for soil behavior. Non-linear push-over analysis and performance evaluations are obtained by SAP2000 computer package. Elastic-plastic Winkler soil behavior is represented by equivalent frame elements subjected only to axial forces and equivalent axial stiffness is defined for these elements. Mat foundation and footings are idealized as linear-elastic thick shell elements in the analysis. Performance evaluations and comparisons for the strengthened building are given in detail for different foundation types.