## 2 Background  
Recent advancements in digital construction have enabled BIM to transform traditional cost estimation into a more integrated, data-driven process. In this context, BIM has transformed cost estimation by integrating cost data with geometric and parametric information, enhancing precision in project planning and execution [11; 12]. Despite recent progress, BIM-based cost estimation remains primarily focused on quantifying material and installation costs. The integration of operational energy costs into BIM workflows is still underdeveloped and inconsistent, thereby limiting its applicability in comprehensive retrofit decision-making [13; 14]. A reliable cost assessment for building energy retrofitting strategies should account for both initial investments and operational energy costs, considering their long-term cost impact to support informed decisions in cost-effective retrofit strategies [15]. Beyond individual cost components, the selection of retrofit strategies also requires the consideration of broader economic implications. Also, given that different retrofit strategies, from partial/standard to deep retrofitting, can considerably impact a building's market value, evaluating a range of alternatives is essential to support informed and value-oriented decision-making [16]. Recent developments have attempted to address these limitations, yet key structural issues remain. The IFC schema provides a standardized framework for structuring cost data within BIM models, improving interoperability between project stakeholders. However, its application in linking energy analysis reports with cost estimation for energy retrofitting projects is still limited [4; 17; 18]. In current IFC-based models, cost data and energy reports are often stored as basic descriptive properties instead of structured entities, which reduces their effectiveness in supporting efficient data access and scenario-based analysis. As a result, stakeholders face challenges when comparing multiple retrofit strategies. Moreover, there are no consistent methods to embed operational energy costs alongside material and installation costs in IFC models, limiting their role in comprehensive cost estimation workflows [9]. A more structured IFC-based approach is needed to improve consistency and clarity in cost evaluation for energy retrofits. Integrating cost estimation within BIM using the IFC Cost Schedule framework improves cost planning and transparency in retrofitting projects [14]. By embedding cost and energy-related data directly into BIM elements, stakeholders can conduct quantitative assessments of different retrofit scenarios [19; 20]. Recent research has shown that BIM-driven cost structuring enhances cost forecasting, better aligning cost estimation assessments with energy-related investment decisions [21]. BIM has the potential to enable the systematic comparison of retrofit scenarios when it is supported by parametric energy simulations. These simulations help estimate the energy demand of each retrofit scenario, forming the basis for evaluating the cost of operational energy [22; 23]. For example, studies on envelope retrofitting show that BIM-linked cost and energy assessments help optimize insulation strategies by combining energy data with structured cost data [21]. Furthermore, recent research highlights the benefits of integrating construction and operation phases within a BIM-based framework to reduce life cycle costs, demonstrating how early stage decisions can impact long-term cost performance in commercial building retrofits [24]. In support of this perspective, Life Cycle Cost (LCC) analysis has emerged as a complementary method to BIM-based cost estimation. LCC facilitates early stage decision-making by combining material, installation, and operational energy costs into a unified cost assessment over the building's life cycle.