6×6 Block is a 35-home apartment building in Girona designed to cut whole-life carbon while keeping comfort low-tech and resilient. The above-ground structure is built with cross-laminated timber (CLT) panels, turning the superstructure into a carbon store and reducing embodied impacts compared with conventional construction. The system is highly rationalised and prefabrication-friendly, limiting waste on site and simplifying future maintenance and adaptation. Operational energy is reduced through a clear passive strategy: every dwelling is dual-oriented (north–south) to enable reliable cross-ventilation and balanced daylight. High insulation minimise winter losses, while summer comfort is achieved without conventional air-conditioning. A habitable gallery on the south façade acts as an “adaptive skin” with two seasonal modes. In winter it works as a wintergarden, capturing solar radiation and passively preheating the intake air for a balanced double-flow ventilation system; in summer the airflow circuit is reversed and the same space becomes a shaded, outdoor-like “shadehouse”, with fresh air drawn from the cooler north façade. Ceiling fans provide an additional comfort buffer during heatwaves, improving perceived temperature with very low electricity use. Beyond energy, the building supports long-term sustainability through flexibility: each flat is organised as six similarly sized rooms (about 12 m²) that can be connected or separated with sliding panels, extending service life by accommodating changing household needs and even occasional shared or communal uses. Together, timber construction plus an adaptive, ventilation-led envelope delivers a durable, low-carbon housing model for a warming Mediterranean climate.