In Korea, energy consumption within apartments in metropolitan areas accounted for more than 33% of the total energy consumption by buildings in 2020. In this study, in order to increase the energy efficiency of MDU (multi-dwelling unit) complexes in metropolitan areas, improvement factors and economic effects were analyzed using ECO2, a building energy efficiency evaluation program. Optimal improvement measures are proposed, to reduce the economic burden on users by applying energy saving technologies. This study was conducted in four stages; in the first stage, using ECO2 software, five types of apartments were selected as standards among 46 complexes. Standard MDUs were selected if more than two factors were satisfied from among the following: (1) household type, (2) average exterior wall insulation and window performance, (3) average energy consumption and demand per unit area per year, (4) average applied facility system, and (5) average monthly energy demand per unit area. In the second stage, improvement factors were derived by analyzing the 10 most recent energy efficient MDU complexes. The third stage involved analysis of the energy saving effect generated by the improvement of windows and total heat exchangers in five selected complexes. Primary energy consumption per unit area per year improved from 158.8 to 132. kWh/m2y in complex E, which had been upgraded from ‘floor heating system’ to ‘total heat exchanger’. Finally, in the fourth stage, optimal improvement factors were selected for economic analysis. By simultaneously applying the optimal improvement factors, such as windows and total heat exchanger, to the M complex, primary energy consumption per unit area per year was improved from 147.6 to 111.4 kWh/m2 y. When optimal improvement factors were applied to 59 m2, 74 m2, 84 m2 types in complex M, life cycle cost savings of energy consumption for 30 years became $1384~1970.