Korea’s Earthquake Disaster Response System currently estimates building damage and casualty figures by applying ground motions computed on a 2 km grid to building statistics aggregated at the administrative district level. While operationally efficient, this methodology lacks the spatial resolution necessary to assess damage at the level of individual facilities, produces discontinuities across grid boundaries, and complicates both data updating and model replacement. In response to these limitations, this paper introduces a geographic information system (GIS)-based earthquake damage estimation platform that adopts individual buildings as the fundamental unit of analysis, developed in collaboration with the National Disaster Management Research Institute. The system manages a nationwide database encompassing approximately eight million buildings, sourced from the national building register, and assigns building-specific fragility functions based on registered structural attributes. Seismic ground motion, damage, and casualty estimates are computed on a per-building basis, with the ability to aggregate results at any spatial scale specified by the user. The platform’s default configurations replicate those of the existing system; however, attenuation relationships, site characterization data, ground motion amplification models, and fragility parameters can be flexibly modified. Analytical outputs are accessible through iterative computational runs, spreadsheet reports, and interactive web map visualizations. The paper summarizes verification efforts against the current system, as well as a case study reproducing the 2017 Pohang earthquake. Four application scenarios on the Pohang testbed—sensitivity to seismic source parameters and model selection, comparison of site database alternatives, identification of high-risk structures, and targeted damage prediction for critical facilities such as schools and police stations—illustrate the enhanced informational value that building-level estimation adds confers for disaster response and preparedness. Comparative analysis with international systems clarifies the distinctive features of the proposed approach.