The transition to a circular economy aims to reduce the consumption of raw materials and minimize waste. To achieve these goals in the context of the built environment, it is crucial to conduct a building circularity assessment (BCA) and select an optimal building design that aligns with both circularity and sustainability objectives at the early stages of design. These objectives can sometimes be conflicting. The emergence of building information modelling (BIM) helped in decision-making processes. However, there is a lack of circularity information within BIM models so there is a need to extend BIM by semantic enrichment. The current BIM-based BCA tools are still in their infancy, use different calculation methods, and do not provide a comprehensive assessment for decision-making support effectively. To address these gaps, a literature review was conducted to identify the circularity and sustainability indicators. Based on the review, a BIM-based framework for simultaneously assessing circularity and sustainability was developed using the identified indicators and employing multiple-criteria decision analysis methods. This paper presents a prototype system that implements the proposed framework using C#, Application Programming Interface (API), and a database. The presented tool functions as a decision support system for the comparison and trade-offs between different aspects (technical circularity, environmental, and economic) of different design options during the early design stage. The aim is to select the optimum solution among design options for a more circular and sustainable alternative. This prototype not only automates the continuous assessment in real-time and supports decision-making but also generates material passports and provides a 3D visualisation of circularity and sustainability performance.