Due to climate change, the built environment is facing increasingly strict environmental targets. Thus, architects are challenged to design evermore high-performing buildings, a task for which they can no longer depend solely on their experience and intuition. Building performance simulation (BPS) tools have become central in this context to support the design process. Yet, several studies show that such tools are still not widespread among practitioners at early design stages. Despite significant efforts made to deliver more “architect-friendly” tools, a gap remains between the expected use and the reality, highlighting the need to adapt the design-approach when developing such tools. A usercentred design approach seems promising for increasing the usability and acceptance of BPS tools, and should be fine-tuned through multiple iterations between BPS developers and potential users via usability assessments. However, as usability assessment has its origins in the domain of human-machine interaction, no methodology has been proposed yet specifically for BPS tools. This paper is the result of a first interdisciplinary pilot study, describing and evaluating a usability assessment method for a new BPS tool that supports the low carbon building design process. Usability, the reliability of the tool and its usefulness are amongst the dimensions that have been assessed with a selected population of future users. Moreover, recommendations and guidelines for the reproducibility of the test are provided. The study shows that both, the quantitative and qualitative results gathered through a usability assessment are insightful to develop a BPS tool that is efficient, satisfactory, pleasant to use and widely adopted by designers.