Sustainability assessment tools and certification systems generally assess environmental impacts of buildings in use phase. Nevertheless, the tools and systems often ignored the interaction between building construction process and the environment. However, we should take environmental impacts of buildings into account during design process not only considering use phase but also by making decisions related to construction activities. In this study, the author introduces an approach for external wall systems, which aims to reduce environmental impacts of construction process by taking decisions in design process. Within the scope of the method, the author developed a database based on a proposed assessment system for selecting and comparing construction techniques of external walls to reduce environmental impacts of construction process.