Water Distribution Network enlargements are caused by growth of demands inside the network and by geographical growth of the network itself. For a water supply system operator, it is very important to have optimum designs of such enlargements and carry these works out at a minimum cost and in a way that makes it possible to satisfy the new demands without deteriorating the service in terms of continuity and minimum required pressure. This paper presents a new methodology for the optimized design of Water Distribution Network enlargements; it is based on the application of two concepts that are related to the degree of vulnerability of the network: resilience and unitary power. This methodology allows to find optimum supply routes and appropriate diameters in order to convey the needed flow from the supply sources to the enlargement areas. Furthermore, the obtained designs were analyzed in terms of costs and were found to be close to the economic optimum designs achieved by means of artificial intelligence techniques.