To achieve the national CO2 emission reduction target (CERT) in the building sector established together with the launching of POST-2020, various countries are introducing the emission trading scheme (ETS), which is considered to have a considerable effect on CO2 emission reduction. Towards this end, it is important to establish a reasonable CO2 emission benchmark for the effective allocation of CO2 emission allowances. As the previous CO2 emission benchmark, however, was focused on the industry sector (e.g., power generation sector, manufacturing sector, etc.), it is difficult to apply to the building sector. To solve this problem, this study aimed to develop a CO2 emission benchmark for allocating CO2 emission allowances in multi-family housing complexes (MFHCs). This study was conducted in three steps: (i) establishment of the database; (ii) formation of clusters using a decision tree (DT); and (iii) development of the CO2 emission benchmark for MFHCs. The nine CO2 emission benchmarks (i.e., 0.03116-0.06667 tCO(2)/m(2) year) for MFHCs were developed using a DT based on the heating type and the elapsed years, and were validated using the Kruskal-Wallis test and t-test. It was shown that using the developed CO2 emission benchmark for MFHCs to calculate the national CO2 emission reduction in MFHCs satisfied the national CERT (18.1%). On the other hand, when the CO2 emission benchmarks for MFHCs calculated based on the South Korean ETS and the EU ETS, which were applied to the industry sector, were used, the national CO2 emission reduction was 5.29 and 45.55%, respectively. The proposed CO2 emission benchmark for MFHCs may be useful for policymaking for determining the allocation of CO2 emission allowances for achieving the national CERT. (C) 2017 Elsevier B.V. All rights reserved.