This study aimed to develop an integrated multi-objective optimization (iMOO) model for establishing the low-carbon scenario 2020 to achieve the national carbon emissions reduction target (CERT) (27.0%) for South Korea׳s residential building sector. This study was conducted in six steps: (i) analysis on the energy consumption patterns of the multi-family housing complexes (MFHCs) by cluster; (ii) establishment of the energy retrofit models for the target MFHC through energy simulation; (iii) scale conversion from the target MFHC to the standard MFHCs by cluster; (iv) life-cycle economic and environmental assessment on the energy retrofit models of the standard MFHCs by cluster; (v) trend analysis of the energy saving effects for the energy retrofit models of the standard MFHCs by cluster; and (vi) establishment of the low-carbon scenario 2020 for achieving the national CERT by using an iMOO model. The results showed that if the optimal low-carbon scenario 2020 is to be implemented as of 2012, the energy-saving effect as of 2020 would be 27.20%. This was determined as follows: (i) the initial investment cost was $1,410,252, 102.5% of the optimized value (optimization goal A); (ii) NPV40 (net present value at year 40) was $18,954,210, 71.9% of the optimized value (optimization goal B); and (iii) SIR40 (saving to investment ratio at year 40) was 5.03, 74.2% of the optimized value (optimization goal C). The proposed model may be useful for policymakers in determining the optimal low-carbon scenario 2020 for achieving the national CERT under the budget limits, from the macro perspective.