Nest City Lab is the adaptive reuse of a former foundry in Barcelona’s 22@ district into a 1,500 m² “urban laboratory for sustainability” that achieved LEED Platinum, with Societat Orgànica supporting the design pase and certification process. Instead of treating sustainability as an add-on, the project turns an industrial shell into a daily-use ecosystem that reduces demand, closes resource loops and restores biodiversity. Embodied impact is tackled first: the existing structure and heritage elements are kept and upcycled, with recovered metal reused on site and most new materials sourced within roughly 800 km, including FSC-certified timber and extensive second-hand furniture and equipment—supporting a circular, low-waste fit-out. Construction and day-to-day use prioritise waste recovery and on-site recycling. Thermal performance is upgraded with breathable, low-impact insulation such as expanded cork on exposed façades and interior linings, lowering energy use while improving comfort. The roof is transparent and openable, creating under it a breathable and dynamic ynamic in-between space. Water strategy is also ambitious for an urban retrofit. The space is designed to draw only about 30% of its water from the municipal network; the rest comes from rainwater harvesting, an on-site well, and reused wastewater naturally purified in the gardens through phytodepuration of greywater from kitchens and showers, then reused for toilets, irrigation and cleaning—sending only blackwater back to the sewer.