One out of eight people worldwide struggles with mental health illnesses. The treatment is slowly shifting from a biomedical perspective, primarily paying attention to symptom reduction and medicine, towards a more holistic and recovery-oriented perspective. At the same time research shows that the outdoor environment can help relieve pain and stress and improve mental well-being. The study presents a practice-oriented method, inspired by evidence-based design, for developing a master plan for two outdoor gardens at a Mental Health Center in the Capital Region of Denmark. We aimed to provide guidelines for how to set up and perform a flexible design process that combines established knowledge with the needs of the patients and staff. The empirical data were collected through a systematic literature review, semi-structured interviews with patients and staff, workshops and presentations at two different mental health units. The two case studies showed good results in involving patients and staff in the design process and based on the findings of our study involving users. In conclusion, it was apparent that the use of interdisciplinary collaboration informed the design outcome. Based on the two cases, the following advice can be given to the architectural profession: Architects should integrate interdisciplinary knowledge into the design process of outdoor facilities for people with special needs. Interdisciplinary knowledge and user involvement complement each other by adding important site-specific details from the user to the more general design guidelines. This way, the design becomes more nuanced and customized to the users of a specific site. The study showed how architects can import knowledge, skills and values from other disciplines such as environmental psychology to improve the decision-making process of future projects for the benefits of mental patients.