Arguments about sustainable urban form have generally been in normative terms without recourse to its practicality. The paper shows that the essential elements of urban form are outcomes of real estate markets. The focus of the research is to examine the economic sustainability constraints to the adaptation of the existing urban form via housing market development viability. To address the task a number of econometric models are linked together to estimate spatial patterns of viability in five cities. The results demonstrate a substantial difference between cities that can be attributed not to urban form per se but to socioeconomic factors. This demonstrates that in practice it is impossible to divorce the physical structure of cities from their economic and social structure. Viability is also influenced strongly by public policy through the location of social housing. The research suggests that a driving force/constraint for development viability is the level of neighbourhood house prices. Large swathes of negative viability are found even without accounting for the additional costs of brownfield development, suggesting that there are major constraints to the reconfiguration of housing markets in some cities in a piecemeal way. © 2009 Pion Ltd and its Licensors.