The construction industry, accounting for a significant portion of energy consumption and greenhouse gas emissions, is pivotal in achieving Europe's ambitious climate neutrality goal by 2050. Circular Economy (CE) principles and the renovation of existing buildings are identified as promising strategies to reduce raw material and energy consumption. However, the lack of knowledge and guidelines for effective CE design and construction in the built environment, along with heterogeneous metrics and standards, pose challenges. The research outlines an investigation study aimed at developing Key Performance Indicators (KPIs) for evaluating building products based on CE principles, focusing on façade renovation. The study emphasizes the need for a holistic approach, considering both material input and output flows, and introduces qualitative and quantitative KPIs addressing aspects such as recyclability, modularity, and local materials. The research proposes established frameworks like Life Cycle Assessment (LCA), Material Flow Analysis (MFA), and the Level(s) framework, but recognizes their limitations in assessing circularity comprehensively. The methodology involves a comprehensive analysis of material streams and circularity potential for nine components crucial to achieving a net-zero façade renovation. Results from the material flow analysis demonstrate the environmental impact of selected building products, such as insulation panels and photovoltaic panels. The research underscores the importance of informed design choices, leveraging adaptable KPIs, and visualizing resource flows to enhance decision-making for sustainable construction practices aligned with CE principles.@en