Recently, Korean government is successively driving energy saving policies. Nevertheless, the use of energy is continuously increasing, and shift toward a low-energy society is somewhat out of our expectation. The major goal of building energy retrofit is to optimize energy consumption in existing buildings which are under deterioration. Many of these old buildings with lower efficiency spend more energy than newly built ones. Thus, this paper suggests a reasonable guideline for energy retrofit. A government office building located in Kangdonggu, Seoul is selected as the subject building, and a modified bin-method called BEAT(Building Energy Analysis Tool) program is used for the analysis. The primary goal of this research is to find the best combination of influencing factors that can improve energy efficiency in old buildings. Such factors stem from design components of Building, System and also Operational facets of a building. By applying the input standards from