This paper will describe a unique cross-disciplinary collaboration between the Medical School at Imperial College London and the School of Architecture and Cities at the University of Westminster, which explored the relationship between design, mental health and wellbeing, using a co-design paradigm. The collaboration included 650 students from across six courses, four under-graduate and two post-graduate namely; BSc Medicine, BA Architecture, BA Interior Architecture, BSc Architectural Technology, Master of Architecture, RIBA Part III as well as 40+ members of staff. Three years in the planning, this innovative project comprised of two one-day co-design workshops, during which 64 cross-disciplinary groups of students reflected on four defined mental health conditions to identify problems and propose solutions for the design of four existing NHS mental healthcare sites. The terms co-design, co-creation, and co-production all describe an open design process that empowers a wide range of stakeholders to make a creative contribution to the formulation and solution of a problem. Co-design offers an immersive and experiential learning experience, and challenges the conventional pedagogy of designer-client / expert-user, introducing experts by experience. While acquisition of discipline-specific skills and competencies was an expected outcome, the structure of the workshops fostered consideration of broader qualities such as ethics, empathy and duty-of-care. This paper will describe both the workshop itself and an evaluation of feedback from students, staff and other stakeholders. http://www.openstudiowestminster.org/co-production-2020-2021/