In construction many problems exist that lead to poor quality of the built environment, for example, lack of integration between service and facility design and poor requirements management. Aiming to mitigate these problems, academics and practitioners have engaged in debating ways of improving the process of designing new facilities. One of the approaches that has been investigated is the use of scientific evidence to support decision makers within the design process. This process is called evidence-based design (EBD). In this respect, a range of studies developed in healthcare sectors encourages the application of EBD in order to add value to the design of these environments and to produce better building outcomes for patients and staff. In this respect, this study was designed to address this issue and is aimed at better understanding how evidence supports design. The achievement of the aim was based on (a) revisiting the philosophical debate about the definitions of evidence and knowledge formation to propose a conceptual framework that can be used to classify evidence within the design domain; (b) investigating the proposed use of evidence within prescriptive design methods of design; (c) understanding how evidence has been used in design practice (specifically in the design of healthcare facilities) and to propose a taxonomy for different categories of evidence that support building design and their advantages and disadvantages; and (d) exploring the existence of opportunities to improve design practice with a basis on a better understanding of evidence. The focus of the research was the design process of healthcare facilities and the unit of analysis was the role of evidence within design. The findings of this research enhance our understanding of design as a knowledge formation system. In this respect, the use of this approach opens opportunities for future studies related to the interpretation and the development of tools that assist design. This research also provides insights related to analysis and synthesis as the proto-theory of design as well as distributed intelligence in design