Inefficiency stands as a major problem in construction industry. Lean thinking is a relatively new process improvement philosophy which approaches inefficiency by reducing the waste. Although the inefficiency of design stages has been associated with the poor results of construction projects, less attention has been paid on the relationship between lean thinking and architectural design process. While there is a strong potential for improvement in the design processes via application of lean thinking, an analysis of waste items has never been done. Thus, this study aims to identify, categorize, and rank the waste items in architectural design sector. In the first stage of the study 28 design waste items were identified and classified under 8 lean production waste categories as a result of semi-structured interviews conducted with senior architects performing in Ankara, Turkey. In the second stage, a questionnaire was executed over a larger population of architects to examine the "frequency of occurrence", "impact over cost", "impact over duration", and "impact over quality" for each design waste item. The findings were statistically analysed and an evaluation of risk was completed. The statistical analysis of the questionnaires shows that there is a shared understanding of impacts of waste items over architectural design processes, with strong statistical evidence over their validity. The results of the study can be utilized for the strategical implementation of lean production philosophy in architectural design process.