The traditional way of supplying energy to the built environment is no longer suitable: New buildings with high energy performance and decentralised renewable energy generation, together with the desire to become fossil-free, involve the need for new, more flexible and more integrated energy systems. The district of Buiksloterham was a test case to develop feasible and potentially desirable energy supply scenarios for the built environment at district level. It is not possible to develop Buiksloterham, and similar areas with high density, into an energy neutral area within the current legal framework (without wind energy it is not possible). About 1/3 of the energy use in buildings (building-related and user-related) can be supplied by renewable energy. In Buiksloterham a low temperature supply of heat is essential for a maximised use of renewable input. A fourth, low temperature, energy concept, consisting of local heat generation from solar and waste, thermal storage, and heat pumps, seems the best integrated energy system. The non-technical lesson learned is that new energy-efficient energy systems require very good, early planning, appointments, and cost and support of existing energy suppliers. Extracting a CO₂ neutral society by 2050 also depends on implementation aspects i.e. not only CO₂ and costs but also circularity parameters such as the use of resources for equipment, water, biodiversity, health, adaptability and resilience must be considered.@en