I Indonesia has a high seismicity potential and a higher response spectrum because the earthquakes that occur tend to be stronger. Therefore, proper handling is required in the design and construction of building structures. The author conducted research on Building X, which was built in 1984. The plan earthquake is determined as an earthquake with a probability of exceeding its magnitude during the life of the building structure of 50 years is 2 percent (SNI 1729: 2020). Currently, Building X is 39 years old and has a remaining building life of 11 years. The author will compare the building performance of Existing Building X based on SNI 1729:2020 if the building is demolished and rebuilt with old materials. This research aims to compare the building weight and the increase in lateral forces that occur in existing buildings based on PPBBI 1983 and SNI 1729: 2020. The method used by the author is by collecting primary and secondary data, modeling the design structure of the existing building based on PPBBI 1983, evaluating the dimensions of steel profiles, and designing the building based on SNI 1729:2020. From the results of the two designs, an increase in building weight and an increase in lateral force were obtained in the design based on SNI 1729:2020.