Urban planners designing masterplans for smart neighbourhoods and smart cities face many challenges dealing with complexity and requiring expertise in various domains at the same time. Simulation models could be used to offer decision support and a tool to evaluate different scenarios and designs. This way the masterplans move from being a static design to an interactive and dynamic test bed. Agent-based modelling has been used to create a synthetic population for a new urban area which can be linked directly with GIS based designs. In two workshops this idea has been tested through the iterative evaluation of masterplans created by architecture students for the Old Oak Common site in London. Here cases on mobility and land values are shown as illustrative examples. The final discussion focuses on city data and the impact of simulation on the work of architects, as well as collaborative decision making and consensus building using simulation tools.