Researches have been conducted to involve the sustainability of energy in the building's early design stage. The methods used include the IEF-based method, the BIM-API-based method, and other approaches. However, these methods have not helped architects out in the early design stage since the complexity of the assessment is not decreased. The contradiction of increasingly complex energy simulations and early conceptual designs forms a research gap in energy optimization. As changes in the early design stage have more effect on the buildings and make less cost, the introduction of on-the-fly energy assessment in the early stages is of great importance. This thesis developed a framework to help boost energy efficiency in building design. The framework aims to improve the efficiency of energy assessment by providing simultaneous and on-the-fly energy performance assessment of the design. BIM software Revit and the visual programming environment Grasshopper were integrated via Rhino.Inside, a bridge between the BIM environment and others to perform the assessment. The workflow of the framework contains three parts: data input from BIM models and the determination method NTA 8800, the connection by Rhino.Inside and the computation of the Grasshopper script, the final results, and the report produced by the Grasshopper script. A script in Grasshopper as a demonstration of the framework was developed as such that Revit models can be directly linked to the energy analysis, and the results can be returned simultaneously. The script was created based on a preliminary case study and two sets of studies were conducted to validate it. The case study of a tiny house was first investigated to test and validate the developed tool. The energy demand result of the developed tool has only a 3% difference for the case study calculated via Uniec 3, a verified software used in the Netherlands. The developed tool was also validated through 4 variants with different configurations to find out if the tool is accurate and robust enough concerning dimensions and physical properties. The comparison of the results shows that the developed tool can produce steadily accurate results that vary within 5% from the standard outcome. In addition, the study indicates the framework's potential of optimizing the building's performance on energy efficiency as the proposed framework significantly shortens the time and workload compared to Uniec 3 which requires a body of manual input. The study in this thesis integrates the BIM models and energy analysis to enhance the sustainability concept in the building design industry. It fills the gap in the application of semi-automated BIM to the BEM framework in the early design stage of buildings. The developed framework provides a useful tool and practical benefits to the field of energy efficiency. This study investigated the potential of applying on-the-fly energy assessment in early design stage, which could be developed further in future work to explore more or better solutions.