Urgent societal challenges have led to unease in our socio-cultural interactions and the production systems that underpin our lives. To confront such challenges, collaboration stands out as an essential approach in accomplishing joint goals and producing new knowledge. It calls for interdisciplinary methodologies such as co-design, an approach capable of bridging multiple expertise. The core activities of co-design are based on the premise of collaboration and on developing creative social environments. Yet achieving collaboration through co-design is challenging as people need to understand each other, and develop trust and rapport. We argue that 'informal-mutual learning' is central to building mutual understanding. This article explores how we create spaces for collaboration through co-design by examining the social environments supporting them. It examines the value of collaboration and its impact upon participants within an action research project conducted in Scotland. We identified Cultural-Historical Activity Theory (CHAT) as a suitable theoretical framework. It offers support for holistic inquiry into participation and learning. Its strength was in the attention that it pays to multi-dimensional human interactions within the social environment. This led to an understanding of the concepts of boundary-crossing and boundary space examined through a CHAT lens. The findings shed light on four designerly conditions supporting informal-mutual learning when engaged in collaboration during co-design situations: choreography and orchestration, aesthetics, playfulness, and quality and quantity of participation. The findings enable us to elaborate on the theorisation of boundary space, a theoretical space for the assemblage of multiple levels of expertise to achieve collaboration.