This study investigates the effects of adjacent deep excavation on the seismic performance of buildings. For that purpose, the numerical models are constructed for different buildings (i.e., 5-Story building and 15-Story building) considering the deep excavation-soil-structure interaction (ESSI) and soil-structure interaction (SSI). The results achieved from the ESSI and SSI systems are discussed and compared. Fully nonlinear numerical models with material, geometric, and contact nonlinearities are developed. Eleven earthquakes with different intensities, epicentral distances, significant durations, and frequency contents are applied to the models; and, the numerical results are given in terms of average records. The buildings are carefully designed and verified based on common design codes. The numerical modelling procedure of the deep excavation-soil system is validated using centrifuge test data. The comparisons between the ESSI and SSI systems are carried out in terms of accelerations, lateral displacements, inter-story drifts, story shear forces, and the nonlinear behavior of the soil medium under the buildings. The results show that it is necessary to consider the ESSI effect, and it might significantly change the seismic behavior of buildings adjacent to the deep excavations. The findings from this study can provide valuable recommendations for engineers to design buildings close to deep excavations under earthquakes.