The metropolis can be analysed as an infrastructure in perpetual motion, composed of built elements – the system “hardware” – and social networks – the system “software”. The emergence of an overall city-scale infrastructure composed not of a single unit but of intermeshed projects designed by various players has given rise to an observable phenomenon of superdensity. The densification created by the development of these large-scale designs at intermodal hubs is generating new typologies, methodologies and completed projects in response to contemporary urban density. Conceived as an ensemble of services rather than as a built framework, these complex projects generate a paradox between the intrinsic stability of their construction and the instability of their functional programme. This is building within the built environment. Metropolitan development via complex design of this kind involves the production of architecture over another architecture. The city regenerates itself over itself: this is building over the built. Functional obsolescence, infrastructural deterioration, and changing mobility typologies generate the possibility of rethinking the potential of what has already been built. The built sites have been repurposed several times over. This means rethinking the accelerated modification of modes of using buildings and infrastructures by reconfiguring – at various intervention levels – projects from the 1980s and 1990s. How is it possible to intervene yet again without this intervention being the last? The process involves finding new resources for optimising an urban fragment within the existing urban fabric, in places divested of their original purpose. This is rethinking obsolescence through a new urbanism. Optimising the choices available to “building sites of the future” means finding new tools for rethinking dense urban environments. This means reconfiguring a neighbourhood by re-using what already exists, without resorting to façadism or simply gutting existing structures. In an age of “cybercities”, of networked megacities, “open core” development sites need to be created – as a way of facilitating the city’s fragmented transformation via the overlayering of decisions and interventions. The scale of the neighbourhood as urban fragment offers us an entry point for rethinking the existing urban cybercities, by applying tools adapted to a single project comprising various operational sub-ensembles. Unique in terms of their scale and complexity, these designs offer an illuminating field for developing research investigating specific aspects of the architecture profession’s future direction in the 21st century. This is designing complexity.