Abstract Climate change urges water managers in low‐lying deltas to diversify their flood risk management (FRM) strategies. To reduce residual risks, they increasingly need to incorporate spatial and other measures. This has implications for the boundary judgements made by water authorities, that is, the implicit and explicit decisions about who and what is relevant to include and consider. To understand these implications, we assess the boundary judgements made by a Dutch regional water authority in two diversification‐oriented frontrunner projects. We distinguish between three categories of judgements: (1) substantive: the scale, domains, time horizon and solutions that are considered; (2) participation: who is involved, to what extent and when; and (3) planning and decision: the flexibility of responsibilities, financing, planning and decision‐making. Our results show that, in both projects, most of the boundary judgements became wider over time as a result of pressure from or interactions with actors from outside the water sector. Hence, despite its ambition to diversify flood risk strategies, the water authority continued to draw boundaries that were too tight to allow for meaningful collaboration with actors outside the water sector. Considering the importance of reconfiguring practices in transforming FRM, we recommend more engaged research into practices.