This paper describes the development and application of a new unique tool to support designers to optimise the sustainability of urban neighbourhoods (SUNtool). In this the paper introduces (i) the software architecture, (ii) the integrated solver and related innovations in the modelling of radiation exchange, reduced thermal modelling, stochastic modelling of occupant presence and behaviour, and urban plant modelling, (iii) interface design and innovations in building attribution, (iv) results analysis methods. Finally the software is applied to demonstrate its application to the development of urban planning guidelines and also to the design of a masterplan.