To explain how multistakeholder groups organize democratic deliberations about complex sustainability issues, organizational scholars have focused on the key role of deliberative capacity, which encompasses the dimensions of inclusiveness, authenticity, and consequentiality. However, the tensions inherent to the search of these three dimensions have been overlooked. In this paper, we argue that focusing on how spaces for deliberation are designed can help one understand how to manage such tensions. We identified the boundary work practices that shape the design of deliberative spaces and generate deliberative capacity properties in a high-level expert group (HLEG) launched by the European Commission about sustainable finance regulation. Our results show how these boundary work practices help balance deliberative tensions. We advance deliberation studies by conceptualizing deliberative boundary work, explaining how deliberative capacity is spatially generated, and showing how deliberative tensions are balanced. We also contribute to boundary work theory by making explicit the deliberative nature of configuring boundary work and showing its relevancy to regulatory settings.