The very notion of sustainable development – let alone its practical implementation – confronts the professions with serious challenges. For sustainable development is said to require integrated, whole-systems thinking whereas the professions arose out of the Enlightenment and subsequent moves to institutionalise divisions of labour. Sustainable development is also argued to need not only collaborative teamworking but also a degree of self-determination – in the form of people actively participating in making decisions that affect their lives. Conversely professionalism seeks to authorise and legitimate expert (as opposed to lay) judgement. This chapter is used to explore how these contending forces are being played out in the UK in the context of recent changes in the regulation and regeneration of the built environment. It is focused on a specific question. Can built environment professionals move beyond the tyranny of a ‘zero sum game’ approach, where one group has to lose in order for another to win, towards win-win outcomes, where both sides may benefit, achieved through using more deliberative forms of practice?