Knowledge plays an essential key role in the policymaking process for interpreting the available information, defining policy issues at stake and evaluating possible solutions – especially in complex policy issues like water management. However, for city-regions, knowledge is often a scarce resource due to the small size of the policy community, context-specific issues, limited availability of resources and experts, as well as the challenge of addressing complex issues that are often supra-local. This paper explores which patterns of local knowledge promote policy change and learning. Starting from the ‘policy paradigm’ concept, a cognitive–evolutionary approach is applied to analyse Brussels’ water management policy, which aims to address the major challenge of flooding. The variety of knowledge by local actors, the role of the policy paradigm of the local policymaking community in vetting information and evaluating alternative solutions, and the importance of local governments for retaining knowledge, are the main dimensions to understanding policy change and learning. City-regions benefit from direct contacts between actors facilitating exchange of knowledge, while supra-local decisions (e.g. EU directives) and local accidents can also trigger major changes. Based on my findings, policymakers tend to rely on technocratic patterns using already available knowledge, mainly whether decentralisation reshapes the policymaking community. While a technocratic pattern determines only minor changes and institutional instability undermines policy learning, policy entrepreneurs and participative patterns can promote major changes and learning if they are able to engage in dialogue with the dominant policy paradigm.