The demand for user participation in design processes is increasing, and there is a need to formulate guidance on how to involve disabled users and their representative organisations. Their participation contributes an extreme user perspective to the design process. The aim of this study was to develop an empirically grounded framework for user participation in co-design processes, involving the users with wide range of characteristics. The study was theoretically grounded in ‘participatory design’ and ‘value sensitive design’ and used an exploratory study design with online workshops to iteratively collect and analyse data. All participants collaborated on an online Miro-board to identify themes and formulate guiding principles for the framework. We propose a framework consisting of three themes: participation fundamentals, participation ethics and participation practicalities, entailing 11 guiding principles. By applying this framework, the premises, methods and activities in the design process will be accessible to all participants, and user participation in design projects will become more transparent, equitable and easier to implement. It will enable all users, people with disabilities and others, to participate and contribute to a design that can be used by the widest range of people.