Numerous existing reinforced concrete (RC) buildings don’t fulfill current seismic code provisions as they haven’t been designed to satisfy these provisions. Also, due to many environmental factors, structural members such as columns, beams, and slabs are exposed to severe deterioration leading to high-risk levels. In this research, the performance of an existing seven-floor reinforced concrete building in Zagazig city, Egypt, constructed about 50 years ago, without considering the seismic requirements prescribed in the current Egyptian code (ECP-2012) is studied. The studied building suffers severe deterioration in its main structural elements, especially the basement floor slab. SAP2000 software is utilized to evaluate the structural behavior of the building under vertical loads (dead and live loads). The model failed to fulfill the ECP-2012 requirements under the applied loads. After building investigation, a retrofitting technique was proposed through providing the building with new grid beams at the basement floor level as well as replacing the existing basement floor slab with a new one. Moreover, RC jackets were provided to all basement floor columns. Through response spectrum analysis (RSA), the performance of the building with the newly proposed technique was assessed considering a peak ground acceleration equals 0.15 g (seismic zone 3). The model showed a significant enhancement in its response that fulfilled the ECP requirements concerning both strength and serviceability. Utilizing SeismoStruct software, the horizontal capacity of the model was inspected through the pushover analysis to check the efficiency of the proposed retrofitting technique. The model showed acceptable capacity in its two main directions.