'Reuse to Reduce. Architecture within a Carbon Budget. The Case of BioPartner 5'is a plea by Popma ter Steege Architects to embrace the circular transition as a chance for a more meaningful architecture. An architecture that employs the necessary reuse of building materials to rethink our needs, way of building and aesthetics. It is a plea for design shaped by planetary boundaries and the use of a carbon budget as a tool for this. It wants to help shift the discourse on circularity towards design and away from technocracy. 'No longer is a building a monument to one institution or person, and no longer is it a sink of materials. It is rather a celebration of what human beings have made until then, and an opening towards the possibility of new ways of living within the ruins and scaffolding we create today.' – Aaron Betsky BioPartner 5 at Leiden Bio Science Park is the first Paris Proof building in the Netherlands. Popma ter Steege Architecten (PTSA) designed the business building with offices, laboratories, and meeting facilities as a gathering place for the campus. Edited by Jan Willem ter Steege. With text contributions by Jan Willem ter Steege, and Aaron Betsky, Mantijn van Leeuwen, Josse Popma.