Plenty of space is available in cities to live, work and recreate more intensively. Yet we see the built environment expanding outside the borders of cities and villages. Instead, we should use the unused space more efficiently. Towns and cities are porous, like a sponge. The undefined space, the oxygen in the interspace, is an extremely strong tool to meet the construction challenge. Together with raw material scarcity and the climate crisis, this requires new solutions. A sustainable, flexible, circular and above all biobased system is the starting point for the development of MONADS.