Hong Kong’s mental health index has languished below the passing mark for five years, a direct casualty of its oppressive hyper-density. With land scarce as gold, building new facilities is not an option. This research turns to the city’s forgotten gaps—its back alleys. We ask: can these neglected spaces be transformed into healing environments? Through typological analysis, case studies, and meticulous on-site surveys, we identify six alley prototypes and develop a suite of targeted mcrodesign interventions. This framework—applied to two distinct case sites—demonstrates how alleys can be systematically repurposed into a network of therapeutic spaces, offering a scalable ‘social prescription’ for urban wellbeing.