<p>In many developing countries, unplanned vertical extensions of reinforced concrete (RC) buildings are frequently undertaken to address the rising demand for residential and commercial spaces. However, such interventions can significantly compromise structural integrity and seismic performance, particularly in earthquake-prone regions. When these extensions are implemented without adequate structural assessment and strengthening measures, they may critically impair a building’s seismic resilience. This study evaluates the effects of vertical extensions on the seismic performance of RC buildings, in the light of February 6, 2023 Kahramanmaraş earthquakes. Field surveys of earthquake-damaged buildings were integrated with nonlinear pushover analyses to quantify the increase in shear demand associated with vertical extensions. Fibre Reinforced Polymer (FRP) retrofitting was applied to mitigate shear deficiencies in critical columns. For this purpose, comparative analyses were conducted using a representative 3-storey RC building model, to which one and two additional storeys were added. Numerical analyses were performed for a total of ten structural models, both before and after reinforcement. Results indicate that vertical extensions markedly increase the number of columns exceeding their shear capacity, thereby heightening seismic vulnerability. Conversely, FRP confinement effectively restores shear capacity and mitigates the risk of brittle failure. The proposed methodology offers practical solutions for retrofitting vertically extended RC buildings to meet performance-based seismic safety criteria. The novelty of this research lies in its integrated approach that links real post-earthquake field observations with analytical modelling of FRP-retrofitted, vertically extended RC structures. The findings provide a methodological basis for future research aimed at improving seismic safety in similar structural configurations.</p>