BACKGROUND: User involvement and participation in the supervision of the quality of care is an important topic for many healthcare inspectorates. It offers regulators an additional view on quality, increases the legitimacy and accountability of the inspectorate, empowers users and enhancing the public's trust in the inspectorate. To assess the accessibility of the local governmental social domain services the Joint Inspectorate Social Domain in the Netherlands worked together with people with intellectual disabilities performing as 'mystery guests' in an innovative project. This paper describes the findings of the evaluation of this project. METHODS: People with intellectual disabilities living at home on their own may need some help with daily activities such as administrative tasks, raising children, household tasks, managing debts or finding work. In the Netherlands they have to arrange this help at their municipality. The goal of this project was to find out how easily people with intellectual disabilities could get help from their municipality. The participants were equal partners with the JISD inspectors from the beginning: in constructing an inspection framework, in acting as mystery guest with a fictive support request, reported back the results by storytelling. RESULTS: The evaluation of the project showed that the JISD succeeded in their key aspect of the project: the goal to involve people with intellectual disabilities in a leading role from the beginning until the end. Their perspectives and preferences were the starting point of supervision. Pain points in accessibility became clear straight away and gave important insights for both inspectors as municipality professionals. Municipalities started to improve their services and evaluated the improvements with the clients. Furthermore, the impact on the participants themselves was also huge: they felt being taken seriously, valued and empowered. CONCLUSION: Involving people with intellectual disabilities as participants in all phases of supervision ...