WikiHousing Barcelona is an applied-research, full-scale prototype of public housing for young people, built on a small leftover municipal plot in Poble-sec neighbourhood, Barcelona. Rather than treating affordability and sustainability as trade-offs, the project tests how cities can deliver high-quality homes faster, with lower carbon, and with stronger social value. The building provides four public rental flats (one per floor, around 50 m² each) plus a generous shared area, and is conceived as a replicable model for tight urban sites and future add-on developments. Its sustainability strategy is “demand first”. A lightweight, prefabricated timber modular system is assembled on site in a matter of weeks, reducing waste, disruption and embodied emissions while enabling reversible, accessible connections designed for maintenance, adaptation, disassembly and reuse. Comfort relies on passive design: the main façade faces south and benefits from deciduous street trees that shade in summer and allow solar gains in winter, combined with high insulation levels, and high-performance openings. With this balance of gains, losses and natural ventilation, heating and cooling demand is driven so low that the building is designed to operate without conventional HVAC systems. Energy and water loops are also integrated: a rooftop photovoltaic pergola supports on-site self-consumption, domestic hot water is produced with an efficient heat-pump system, and a greywater recycling approach reduces potable-water demand (aligned with the reuse of shower water for toilet flushing). The project is monitored and framed as open innovation, bridging research, construction and training through a participatory “building-as-a-school” process that equips young people with skills while expanding the public housing stock. In spirit, it anticipates upcoming EU performance expectations and resonates with the New European Bauhaus focus on sustainability, inclusion and quality of life.