The Netherlands has roughly 600 square kilometres of flat rooftop space, an area as big as Amsterdam, Rotterdam, and Utrecht combined. Everywhere, cities are densifying, but there is still plenty of room for the issues that brings with it: housing challenges, energy transition, climate adaptation, and greater inclusiveness. Just look up. Existing flat rooftops provide a “hidden” space in the city that for now remains largely unused, but the possibilities are endless. From playgrounds to rooftop farms, dog walking areas to extra classrooms, outdoor fitness to solar panels, there is a rooftop solution for everything. This catalogue details inspiration for thinking about the layered city of the future.