The existing assessment tools for the life cycle carbon dioxide (LCCO2) have certain limitations in that users have to directly enter quantities of materials after the construction document phase and they are likely to find it difficult to formulate an optimal strategy to reduce CO2 emissions. Therefore, Global Environmental Model/Management-21P (GEM-21P) was developed for establishing a responsive and accurate system for suggesting methods for high energy efficiency in the planning phase. The objective of this study is to identify the requisites for an LCCO2 assessment program that is based on the technical elements of GEM-21P and that can be used in the schematic design phase. The life cycle of a building is divided into four stages: material production, construction, operation, and dismantlement/disposal. Further, the technical elements of GEM-21P are examined for assessing CO2 emissions in each of the stages. On the basis of the examination results, we specify the requisites for a system that can assess, in the planning phase, the CO2 emitted from a building during its entire life. Crown Copyright (c) 2013 Published by Elsevier B.V. All rights reserved.