Borders are essential in the current planning of cities since new forms of social relations are needed to support more sustainable ways of life. In this article, we present a case study of a sustainability-profiled new neighbourhood, Vallastaden in Sweden. We focus on how sustainability is enacted in different socio-material versions, which often include defusing borders between private and shared spaces. Shared space in Vallastaden includes spaces to facilitate meetings, such as felleshus (built as semi-communal, ground-level buildings, semi-indoor spaces, and greenhouses), winter gardens (built as rooftop, semi-private, semi-indoor, and social spaces), and the shared brook-park Broparken and farm-park Paradiset with rental allotments and communal gardens. Analysing how bordering practices create inclusion and exclusion, we study their consequences for the everyday lives of humans and non-humans in Vallastaden. We conceptualise these dynamics as fluid and fire space in order to make the ontological politics of bordering visible. Our study shows that the borders in the planned shared spaces are dynamic and create both fluid and fire space, depending on their socio-material relations. The research shows that planners need to take these heterogeneous socio-material relations into account when creating borders because, otherwise, they risk creating unfair exclusions.