Despite efforts to reuse and recycle materials in the building industry, the vast majority of materials used is new, which does not contribute to a circular built environment. Additionally, the way we design influences the amount of waste produced by the building industry. This research aims to develop circular design principles for optimal reuse of building components during the design process. Main factors of availability, dimensions, aesthetics, technical performance, environmental impact and the costs influence the reuse potential. Through qualitative data analysis, including in-depth interviews with pioneering architects who apply reuse as building method, this study identifies design principles and strategies for implementation in the design phase. By starting the design with identifying the reused materials, early collaboration, designing for disassembly and adaptability, and allowing flexibility during the whole process, optimal reuse can contribute to circular building practices.