Housing affordability is one of the main aspects required for sustainable development and society. However, the timely delivery of new homes is often constrained by the need to upgrade and expand essential infrastructure such as water and electricity networks. For water utilities, responses to growth typically involve intensive hydraulic analysis to assess water distribution systems (WDS) capacity, identify upgrade needs and evaluate options for system extensions. This process becomes significantly complex and resource-intensive under high growth conditions, where a higher volume of faster answers is required to address a wide range of uncertain future scenarios. This paper presents a concept of using generative artificial intelligence (Gen AI) integrating with hydraulic models to form an AI Agent to support WDS design. Specific features of Gen AI used within the hydraulic agent are discussed. A real-life case study demonstrated that the AI agent can analyse land development requests, trigger hydraulic simulations and identify augmentation needed, significantly reducing manual tasks. This offers a breakthrough strategy for water distribution system design and planning to enable sustainable water infrastructure development.