The formulation of management plans as required by EU environmental policies such as the Floods Directive may facilitate the uptake of nature-based solutions (NBS) into practice. Previous research has indicated that the uptake of NBS in water management plans is still low and hindered by various elements of the existing water governance system. However, research so far neglected the role of water managers as “plan-makers” of solution strategies and programs of measures, as well as their beliefs in choosing certain measures in the plan-making process. The aim of this study is to shed more light on the plan-makers’ reasoning for integrating, or not integrating, NBS into specific flood risk management plans (FRMPs). We conducted ten qualitative interviews with plan-makers from Germany and adopted a grounded theory approach to identify their beliefs that underlie the process of formulating FRMPs as well as their perceived role in this process. The analysis reveals a dominance of shared substantive and relational beliefs that are obstructive to a greater uptake of NBS in FRMPs. In particular, identified beliefs about NBS often do not align with their self-perception of their role in being the “plan-makers”. We present a differentiated portrait of water managers as key actors in the decision-making on FRMPs, illustrating that while water managers are belonging to the same distinct professional group with a similar social role in the decision-making process, they do not necessarily share the same preferences.