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. The architects developed a design for a demountable building and a host of ideas to reduce the consumption of raw materials, energy, and water. This included a strategy of scouting out potentially reusable materials in the course of the design process and purchasing them themselves. That this would lead to the purchase of the entire main supporting structure from used steel was not foreseen at the time. The new BioPartner 5 laboratory at the Leiden Bio Science Park was constructed from as much as 165,000 kg of steel that had formed the base of a nearby university building for 50 years. The steel was dismantled, moved 750 metres, processed on site, and then rebuilt as a ‘donor skeleton’ for the new building. Through its unique scale and method, BioPartner 5 shows the promise of large-scale reuse in construction. From the entire reused steel supporting structure to the internal walls, from the floor finish to the toilets.