The city of Amsterdam is going to build a new housing estate: IJburg. About 18,000 houses will be built and finally 45,000 people will live in IJburg. An important objective is to realize a sustainable housing estate: the design is focused on water and energy saving, and the use of sustainable building materials. An alternative water supply system, in which household water is used besides drinking water, may be an important contribution to this ambition. Household water can be used for toilet flushing, for washing clothes or for both. Amsterdam Water Supply developed five scenarios for such an alternative water supply system. Water saving and supply of two types of water are key elements in these scenarios. Based on a comprehensive selection in which public health aspects, technical feasibility, legislation, acceptance by the public, costs and environmental impact were used as criteria, the optimum scenario was chosen: a dual water system, supplying drinking water from the existing drinking water production plants, and supplying household water for toilet flushing and washing, produced from a local surface water, the IJ-lake.