Flood risk management (FRM) is moving towards more proactive and collaborative direction to enable adaptation to changing conditions. We present a case study on collaborative planning process, which contributed to the development of adaptive FRM in one of the largest river basins in Finland. The focus was on the possibility and acceptability of using large regulated lakes as storage for flood water in an extreme flood event to decrease flood damage at the downstream riverside towns. We defined an extreme flood event that would cause dramatic flood damage and developed tools for simulating the event with alternative regulation strategies using Watershed Simulation and Forecasting System (WSFS). We organized a stakeholder event to demonstrate the alternative lake regulation strategies, their socio-economic consequences, and to discuss their acceptability. We found that storing flood water in the lakes above the regulation limits and preparing for winter floods in advance by lowering the lakes in the autumn can minimize the total damage in the target area. The majority of stakeholders considered these actions acceptable in an extreme flood event, regardless of deliberately induced flooding of areas where no floods have occurred for over 50 years. However, lowering the lakes in the autumn on annual basis gained less support. We emphasize the importance of deliberations on the FRM procedures and responsibilities in extreme flood events with the stakeholders in advance to increase adaptive capacity and legitimacy of decisions.