Ostrom’s design principles have been broadly used to analyse the governance of common pool resources. However, they are normally assessed as present or absent. We challenge this assumption by considering them as fuzzy sets where membership scores range from 0 to 1, because, in real situations, the design principles can be present at a certain level. We define categories to assess the level of membership and apply it to a single case study analysing how changes in water policy can affect the community-based management of the Water User Association of Mondomo (Colombia). In rural areas of Colombia, most water and sanitation services are provided by water user associations, wherein civil society has developed governance systems based on active citizen involvement and community-based management. Some of these associations have been operating for decades and are essential pillars of the local social fabric. However, recent changes in the country’s policy and legal framework threaten these long-lasting governance systems. The results show that most of the design principles would suffer important changes that undermine the governance system. Essential principles for sustainable community-based governance, such as the congruence of the rules with local conditions, the local monitoring and sanctioning capacities, the internal conflict-resolution mechanisms and the recognition of the rights to organize, are dramatically reduced after the policy implementation.