# A Systematic Review of the Trends and Advances in IFC Schema Extensions for BIM Interoperability
## 1 Introduction  
With the increasing adoption of building information modeling (BIM) throughout the architecture, engineering, construction, and facility management (AEC/FM) industries, its applications are expanding beyond just modeling to include specialized fields, such as quantity take-off, energy analysis, code compliance checking, and structural analysis [1; 2; 3; 4]. The rise in BIM adoption has significantly contributed to the shift from traditional construction project implementation methods towards a model-based collaboration approach, enabling better communication and information workflow [5; 6; 7; 8]. Leveraging BIM model-based collaboration environments has also underscored the need for interoperability of data exchanges among the domain-specific BIM applications used by project stakeholders [9; 10]. The industry foundation classes (IFC), an ISO 16739-1:2018 standard [11], offers a neutral and open data format for facilitating the seamless exchange of information between various BIM applications. The IFC provides a comprehensive schema to represent a broad range of concepts and information about objects, attributes, and relationships in BIM models [12]. This foundation allows it to represent not only physical elements in multiple domains including architecture, mechanical electronic plumbing (MEP), and infrastructure, as well as concepts and processes pertaining to the design, construction, and management throughout the life cycle of a construction project [13; 14]. The IFC data schema is also under continuous refinement to better suit the evolving needs of the industry. building SMART International (bSI), a non-profit, multilateralorganization, presides over the maintenance, upgrades, and distributions of IFC development. The technical committees within bSI strive to expand domains and concepts covered within the schema, in tandem with the growing needs of the AEC/FM industry [15]. However, adding and standardizing new domains and concepts involves a collaborative effort, requiring thorough reviews within bSI committees, followed by alignment with ISO standardization procedures. Meanwhile, in less common domains, using the IFCs required creating custom extensions to the schema as a temporary measure before official updates and upgrades were released [16]. Commensurately, numerous studies have suggested ways to extend the IFC schema to meet the needs of specialized domains or encompass nascent technologies. Extension of the IFC involves defining new physical elements or conceptual ideas and integrating them into the existing schema. It facilitates the inclusion of specific data or exchange requirements not initially accommodated by the base schema. These modifications serve to meet industry-specific or project-specific needs, thereby enhancing the effectiveness of data exchange for a wide range of construction and design purposes [17; 18; 19; 20]. With the AEC/FM industry increasingly reliant on data from multiple domains and upcoming technologies, the need for IFC schema extensions has gained greater importance and urgency. A variety of extension approaches have been used throughout IFC's history; however, a systematic collection and evaluation of these studies remain absent. Although refs. [21; 22] have previously provided literature reviews concerning IFCs, they were limited to introducing the historical developments of the standard schema and did not explore in-depth studies related to IFC schema extensions. Different extension approaches, as well as their implementation and validation approaches, may be used depending on the domain of interest and objective, and yet such distinctions have not been evaluated and made explicit. Formalizations of these distinctions can be used as a valuable guide into how future extensions should be made. This study collected and analyzed 64 cases pertaining to extending the IFC schema over the past two decades, from 2001 to 2022.