Tuned Mass Dampers (TMDs) are traditionally employed in tall buildings to control displacements, typically featuring a low mass ratio. This study explores the potential of TMDs with significantly higher mass ratios for seismic retrofitting in existing civil structures, an area that remains largely unexplored. The research investigates non-conventional TMDs with exceptionally high mass ratios, which can be implemented by either elevating the existing structure or utilizing a part of the building's superstructure as a tuning mass on seismic isolators. A comprehensive parametric analysis was conducted, encompassing various ground motions, mass ratios, and damping levels, to refine the design of these TMD systems. The results, presented in detailed design charts, reveal optimal response characteristics based on the base structure's vibration period, TMD's damping, and additional mass. Notably, reductions in maximum top displacement of more than 30% and decreases in top acceleration of more than 40% were observed in certain configurations. The efficacy of these high mass ratio TMDs was further validated through a benchmark case study of an existing building, analyzed using non-linear Incremental Dynamic Analyses (IDAs). The findings indicate a significant enhancement in seismic performance and an improvement in structural response with the integration of non-conventional TMDs, suggesting a promising direction for seismic retrofitting in buildings designed under outdated seismic criteria.