Improving the climate resilience of urban areas critically depends on the integration of climate adaptation measures, i.e. mainstreaming, into regular construction practices. As research has largely focused on public sector adaptation, the mainstreaming of adaptation into private sector projects remains poorly understood. The aims of this study are twofold. First, we examine what drives private developers and investors to mainstream adaptation into large-scale urban development projects. Second, we explore what policy instruments municipalities can employ to stimulate private sector mainstreaming. Our theoretical lens combines insights from the literature on mainstreaming, sustainable building drivers and policy instruments. These concepts are used to guide our analysis of four urban development projects and an interview study in the Netherlands, a densely populated delta country which is rather vulnerable to the impacts of climate change. Our results show that private developers and investors seldom explicitly include adaptation measures into their development projects. An important impediment is the perceived absence of direct monetary benefits. If adaptation measures are implemented, they are often realized as a side-effect of creating a high-quality living environment or because private actors expect other professional benefits, such as corporate image enhancement or development of know-how. To stimulate private sector mainstreaming, Dutch municipalities already use a mix of policy instruments that might be a source of inspiration for other countries. Yet, especially the way in which enforcement and incentives are applied is not always effective. Key policy insights Despite the private sector’s growing awareness about the need for and importance of climate change adaptation in the Netherlands, adaptation is still seldom explicitly included in large-scale urban development projects. Municipalities should invest in policy instruments that target consumers. Communication, education and incentives can be used to raise consumer awareness and consequently demand for climate-adaptive properties. Municipalities should collaborate with the private sector to develop clear, uniform and feasible adaptation requirements. Municipalities should actively participate in urban development projects, i.e. co-developing with the private sector. This way, private sector drivers and policy instruments can strengthen each other to pave the way for future-proof and climate-resilient urban environments.