A Multimodal Model- and Retrieval-Guided Framework for BIM Model Cost Estimation

BIM model-based construction cost estimation requires reliable linkage between model-derived building information and estimator-facing cost records. However, BIM models and structured cost databases use different descriptive logics: BIM model data primarily describe what a building component is in the model, whereas cost records primarily describe how that component is constructed, measured, and priced. When BIM model names are non-standard or properties are incomplete, this mismatch may lead to ambiguous cost item selection, particularly when candidate records differ in unit basis, material assembly, thickness, finish, fire rating, or performance requirements. To address this problem, this study proposes a multimodal model- and retrieval-guided framework for BIM model-based cost estimation. The framework converts BIM model content into standardized estimator-readable descriptions, retrieves cost database candidate entries, applies rule-based checks for unit, material, thickness, finish, and fire rating consistency, and produces reviewable cost item selections for database-based cost calculation. The method uses a multimodal model to supplement and standardize component information, while cost records remain the authority for unit prices rather than being replaced by model-generated estimates. The framework was evaluated using a BIM example containing 7374 building elements across 21 model element types, together with a structured cost database containing approximately 11,500 pricing records. The full workflow reduced unmatched categories and improved pricing coverage relative to direct cost item retrieval. The results indicated that the proposed method can improve the technical appropriateness and coverage of cost item selection. The study contributes a reviewable workflow that integrates BIM model content, multimodal description standardization, cost database candidate retrieval, rule-based specification filtering, and database-grounded cost synthesis for selecting justified cost items under practical estimating ambiguity.