The use of fractal techniques to model aspects of urban form is now well established. However, to date these have concentrated on specific general attributes of urban form, usually focused at a macro level. In this paper we argue that the urban system is complex and its global spatial attributes related to patterns of activity and movement at the local scale. In order to arrive at a generative model which simulates the pattern of urban development, two interrelated urban spatial systems need to be modelled together. These are the built form system which reflects the pattern of uses and activities associated with the use of particular spaces and the movement system that connects those spaces. We will describe early experiments in spatial modelling which attempt to combine aspects of the two systems in the understanding of the development of informal settlements and low-income housing in developing countries.