The optimal performance of the Adaptive Negative Stiffness Device (ANSD) assisted building system has been proposed to mitigate the seismic responses of 14-story building. In the proposed implementation, the building block above the ANSD-augmented story imparts the effect of a non-conventional Tuned Mass Damper (TMD) (with large mass ratio) on the lower block and is simultaneously imparted with the isolation effect due to ANSD induced flexibility. The ANSD composed of viscous damper in addition to their negative stiffness thereby helps in dissipating the input energy and controls the large displacement in the NSD. The optimal parameters are derived based on the optimal tuning and damping ratio obtained from a state-space analysis, considering non-classical damping. To check the efficacy, non-linear time-history analysis are performed subjected to a number of ground motions (from the SAC project) representing varying degree of hazard levels. Statistical analysis reveals that the proposed system is effective in reducing seismic responses compared to its uncontrolled counterpart.