Since the early 2000s, systems for evaluating the life cycle CO2 (LCCO2) emissions of buildings have been developed in order to reduce emissions and help prevent global warming caused by greenhouse gases. However, existing systems have limitations in that they only support a one-time evaluation of CO2 emissions during the early planning or progress phase and cannot effectively provide feedback on evaluation results. Hence, this study aimed to develop an integrated assessment system that periodically evaluates and manages LCCO2 emissions of a single building according to project processes, consisting of the early planning, progress, and construction phases. To this end, requisite factors for the integrated assessment system were analyzed, and the concept of this system was designed, including models for simple assessment, detailed assessment, construction site assessment, and result analysis. Further, the life cycle stages of buildings were classified into the production, construction, operation, and end-of-life stages, and a CO2 emissions evaluation method and database were established to implement the evaluation model. Through this process, the web-based integrated assessment system was developed, and its practical applicability was verified through a case study.