The functionality expected by governments and citizens from urban water management systems (UWMS) has evolved in time from delivering basic services to enabling complex issues such as healthy ecosystems, environmental sustainability, and economic growth. Alongside these changing expectations, the pressure on policy makers to fulfill disparate performance metrics with diminishing resources is increasing. In this paper, we introduce the Water Assets and Infrastructure Network Decision Support (WAND) framework for modelling and planning urban water systems with the aim of assisting medium to long-term investment decisions. The framework combines economic performance measures with liveabililty and sustainability, and uses optimisation as a crucial component. To demonstrate its feasibility, we present a prototype environmental software system built upon the principles of legibility, adherence to engineering conventions, and extensive unit testing. We use the prototype system to analyse a model of a hypothetical urban water system formed by two coupled water networks, one for freshwater and one for stormwater collection, and which handles six different commodities. Our results suggest to planners the optimal combination of planning investments while considering capacities, service levels and network operating conditions.