This consultancy case study details a project with a UK local authority to develop a behavioural science-informed website supporting self-management for well-being. Commissioned through a successful tender, the project involved co-design to (1) identify user needs for website navigation and content, and (2) determine stakeholder requirements for structure, content, and tools. These findings informed recommendations for the website’s technical specification. We outline some of the challenges faced, and the context and boundaries of the project that were necessary to inform planning. The project involved different stakeholders with competing interests and motivations. This meant that the brief evolved during the project, and our interactions with the client involved a repeated cycle of planning, and appraising changes in project direction. We continued to (re)negotiate around these changes in order to minimise scope creep and ensure that the work we were required to deliver remained of high quality and within the boundaries of our expertise as behavioural scientists. We reflect on clients’ understanding of behavioural science, professional boundaries, shared perspectives, and the distinction between co-design and co-production.