Energy-efficient buildings are essential to reduce greenhouse gas emissions. However, improved energy efficiency may also bring about a rise in construction costs. If design variables can be prioritized according to their cost efficiencies, energy-efficient buildings can be designed more cost-efficiently. The aim of this study is to analyze the cost efficiencies of design variables related to energy efficiency of the apartment complex. After making 78 alternatives for 29 target items, each alternative's increased amount of construction cost and decreased amount of energy cost against the baseline model were then estimated using actual price data and the assessment methods of the Building Energy Efficiency Rating System that has been implemented in Korea, respectively. The life cycle cost, net present value, internal rate of return, and payback period of each alternative were shown as cost efficiency indexes. The order of priority of each alternative was presented and very cost-efficient alternatives were suggested.