Despite adopting a mainstreaming approach to spatial adaptation, few cities actually implement it. Why this is so is disputed, as the implementation stage has been under-researched. Inspired by the “mechanism-based” strand, this paper analyses three implementation projects in the Netherlands. Mechanisms are generalizable patterns yielding more plausible explanations for phenomena than “superficial” barriers or drivers. We found one overarching mechanism: the “institutional void” – an absence of policy frameworks with clear norms and agreements on adaptation. It led to three hampering mechanisms (avoidance of administrative responsibility, aversion to innovation and avoidance of private responsibility) and three stimulating mechanisms (window of opportunity exploitation, efficient coupling and policy entrepreneurship). We demonstrate that, although hampering mechanisms constrain the solution space for mainstreaming adaptation, the stimulating mechanisms prove that room remains for committed actors to use the limited available space efficiently.