Cost overrun is one of the most common problems that threaten any construction project. As the Design phase is responsible for many critical decisions, many of cost overrun causes are related to such phase. This paper aims to identify the most significant causes of cost overrun related to the design phase in the Egyptian construction industry from the point of view of owners, consultant, designers, project managers and contractors. A list of cost overrun causes related to the design phase collected through an extensive literature review, main causes were adapted to the Egyptian construction industry through seven semi-structured interviews. The resultant list was submitted to a questionnaire survey for the impact and frequency quantitative evaluation. the results of the research is expected to help the participants in the design phase to develop more optimized design and avoid the most usual flaws that could led to cost overruns.