As construction industry climate changes rapidly and housing consumers expect higher quality, construction contractors are held more responsible and liable for defect maintenance and defect maintenance costs are rising steadily in response. However, it is true that construction contractors still relies on conventional and unsystematic approaches in planning and budgeting processes. As a result, planned defect maintenance costs by work trade based on conventional process vary from actual costs, resulting in significant opportunity costs as well. Therefore, it is necessary to study potential improvements to apartment defect maintenance cost estimation process. This study is intended to improve apartment defect maintenance cost estimation process. Therefore, such systematic and efficient process improvement research is expected to improve effectiveness in apartment defect maintenance.