Despite its broad definition, the concept of sustainability has become central to regeneration policy in the UK. A growing body of research, however, suggests that the policy goals of urban regeneration and sustainable development are not being integrated in practice. Ambiguity surrounding what ‘sustainability’ actually means is often cited as the reason why projects fail to achieve policy goals. We seek to make an innovative contribution to this debate, arguing that sustainability does make a positive difference in practice, and that it is necessary to develop approaches that capture these ‘actually existing sustainabilities’. Using a detailed case study of a multistakeholder regeneration project, we develop a more positive analysis of the role which ambiguity plays in the development process. We advance a dialogic conception of sustainability based upon Michel Bakhtin's sociolinguistic theory of the word as a ‘shared territory’. We suggest that the notion of sustainability acts as a shared territory for meaning around which diverse stakeholder groups coalesce, and show how the ambiguity inherent in this shared conception can generate more creative (and sustainable) outcomes to developmental challenges. Viewing sustainability as a shared territory makes ambiguity not only intelligible, but also desirable to the development process, and it is argued that there is a need to avoid the reduction of sustainability to the assessment of predetermined benchmarks or policy goals, both within the regeneration literature and across studies of planning policy and practice more generally.