Historically, little consideration has been given to water performance of urban developments such as “hy- drological naturalness” or “local water self-sufficiency”. This has led to problems with increased stormwa- ter runoff, flooding, and lack of local contributions to urban water security. Architectural design, water servicing technologies and environmental conditions are each known to influence water performance. However, most existing models have overlooked the integration of these factors. In this work, we asked ‘ how the water performance of urban developments at site-scale can be quantified, with joint consideration of architectural design, water servicing technologies, and environmental context ( i.e. climate and soil) ’. Answer- ing this question led to the development of a new method and tool called Site-scale Urban Water Mass Balance Assessment (SUWMBA). It uses a daily urban water mass balance to simulate design-technology- environment configurations. Key features include: (i) a three-dimensional boundary focussed on the “entity” of development (ii) a comprehensive water balance accounting for all urban water flows, (iii) methods that include key variables capturing the interactions of natural, built-environment and socio- technological systems on water performance. SUWMBA’s capabilities were demonstrated through an eval- uation of a residential infill development case study with alternative design-technology-environment con- figurations, combining three dwelling designs, seven water technologies and three environmental con- texts. The evaluation showed how a configuration can be identified that strikes a balance between the conflicting objectives of achieving the desired dwelling densities whilst simultaneously improving water performance. For two climate zones, the optimal configuration increases the total number of residents by 300% while reducing the imported water per capita and stormwater discharge by 45% and 15%, re- spectively. We infer that SUWMBA could have strong potential to contribute to performance-based urban design and planning by enabling the water performance of dwelling designs to be quantified, and by facilitating the setting of locally-specific water performance objectives and targets.