An openBIM-Based Integrated Process for Quantity Take-Off and Schematic Cost Estimation to Support Schematic and Preliminary Design Decision Making

Construction projects are inherently complex systems that require effective decision-making during the schematic and preliminary design stages, when major design alternatives are evaluated and a substantial proportion of project costs is determined. Therefore, rapid and reliable cost estimation is critical at this stage. However, traditional estimation methods based on two-dimensional drawings often yield inconsistent quantity take-off (QTO) results due to reliance on manual interpretation. In the context of this study, this stage refers to a schematic-to-preliminary design phase in which major building elements have been modeled at an LoD2 level and can provide approximate quantity information for cost evaluation. Although BIM-based QTO approaches have been proposed to address these limitations, existing studies predominantly focus on quantity extraction rather than the integration of quantity and cost information. Additionally, the reliability of BIM-based estimation depends heavily on BIM model quality and structured data preparation. To address these issues, this study proposes an openBIM-based integrated framework that systematically links BIM, model quality verification, QTO, and schematic cost estimation within a unified workflow. The proposed method integrates IFC-based quantity data with a structured cost database to support systematic cost estimation during the early design stage. This study focuses on building frame elements because of their significant impact on project cost. The methodology comprises three main steps: (1) BIM and quality preparation based on LoD2-level requirements, (2) IFC-based automated QTO, and (3) model-based schematic cost estimation through QTO–cost DB linkage. A prototype system was developed to validate the proposed framework. The results indicate that the proposed approach improves estimation reliability by reducing risks associated with model quality deficiencies and manual processes. Furthermore, the integrated workflow enables faster estimation and supports more consistent cost evaluation for schematic and preliminary design decision-making.