This paper is concerned with the act of co-designing as a research method within doctoral design studies in which multiple participants design together as part of a design PhD research project. Co-design as a research method is an under researched activity and, we argue, there is a danger that the co-design, relying on often implicit, unacknowledged and opaque undertakings, can undermine the rigour of the research process in comparison to other methods that are more completely defined and articulated and comparable. Drawing on an analysis of co-design, we present six key questions to be explicitly addressed before embarking on co-design as part of doctoral research. These intentionally straight-forward questions preserve the essential essence of playfulness and freedom in co-design while offering a framework to help make explicit the assumptions often unexamined in co-design as part of a PhD in design.