The purpose of this study is to establish the assessment baseline of $CO_2$ emissions from building operations in the view of GHG reduction policy in Korea. The assessment baseline of $CO_2$ emissions shall be used in GHG policy and Carbon Credits in building sectors, but the assessment baseline has not been studied enough or established yet. Also, $CO_2$ emissions from building operations will be variable according to the building occupancy. Therefore the baseline will be different and this study aimed at the establishment of the assessment baseline for residential apartments and office buildings firstly. After reviews of BEER and international standards for building $CO_2$ emissions such as ISO and UNEP-SBCI documents, the analysis of BEER certification data has been pursued for 292 residential apartment complexes and 65 office buildings in South Korea during 2004~2012. As analysis results, the assessment baseline was set to 23.03 $kg{cdot}CO_2/m^2{cdot}yr$ or 1.95 $t{cdot}CO_2/unit{cdot}yr$ for residential apartment complexes, and 95.91 $kg{cdot}CO_2/m^2{cdot}yr$ for office buildings according to the BEER certification basis. Additional assessment baselines were calculated according to year basis, region basis, public and private basis, and GHG policy basis. Finally, the established baseline for residential apartment complexes has been applied for the pilot project in M district, Seoul, and showed 24.97% reduction rate according to the BEER certification basis.