Most of the existing reinforced concrete (RC) buildings in India are either designed before the existence of seismic design codes or without proper provisions to resist lateral loads. These structures generally possess low strength and limited ductility, thereby always prone to global failure and are a primary reason for causalities during an earthquake. Hence, seismic evaluation of these structures and strengthening, if necessary, is essential to prevent a disaster during its serviceable lifetime. In this investigation, a four-storied (G+3) RC building structure in Warangal city, Telangana State, India, has been selected as a case study. Furthermore, the structure is to be extended vertically by two stories above the existing configuration. The structural and material inputs to the analytical model are updated from visual inspection and nondestructive tests, followed by an analytical study. From the analytical study, the seismic capacity of the existing gravity load-designed structure itself is found to be inadequate for seismic requirements. Since the structure is to be extended vertically, the existing building needs to be strengthened appropriately to withstand additional loads and remain functional throughout its lifetime. The outcome of the seismic analysis is checked for conformity using demand and capacity requirement of the structural components of the six-storied (G+5) structure. This behavior mimics the current scenario of existing RC buildings in this locality, which needs to be appropriately strengthened. It has been observed from the analysis that the RC building was vulnerable to the seismic forces and needed retrofitting to remain functional during its service life.