Is compliance with the 3-30-300 rule associated with subjective citizens' well-being?

To address urban challenges and climate change, sustainable urban planning is essential for creating resilient and livable cities. The 3-30-300 rule proposes that every resident should see at least three mature trees from home, live in an area with at least 30% tree canopy cover, and be within 300 m of a publicly accessible green space of at least 0.5 ha. However, evidence for its health and behavioral benefits remains limited. This study aimed to: (1) assess compliance with the 3-30-300 rule in a typical Mediterranean city in 2025 (Aix-en-Provence, France); and (2) examine associations between the rule, its individual components, and health and behavioral outcomes using covariate-adjusted fixed-effects linear regression models with CR2 cluster-robust standard errors and health survey data (sampled participants, n = 1195 adults). Cross-sectional analyses examined self-reported physical and mental health, behavioral outcomes (stress, anxiety, loneliness, sadness), and quality of life indicators. Results suggest that residential greenness may improve mental health, sleep quality, and behavioral well-being, while evidence for benefits from meeting the full 3-30-300 rule remains inconclusive. We found significant associations (p < 0.05) between a higher prevalence of buildings meeting at least two 3-30-300 components and better self-rated mental health (β = 0.0054, 95% CI: 0.0010; -0.0097) and lower stress levels (β = -0.0081, 95% CI: -0.0122; -0.0040). Higher district-level visibility and canopy coverage was associated with better sleep quality (β = 0.0059, 95% CI: 0.0017; 0.0101). For behavioral outcomes, higher district-level canopy cover was associated with fewer sadness (β = -0.0066, 95% CI: -0.0125; -0.0006) and anxiety symptoms (β = -0.0073, 95% CI: -0.0159; 0.0014). No statistically robust associations were observed for physical health, loneliness, or district quality of life. Sociodemographic factors, chronic illness burden, mobility patterns, physical activity, and perceived environmental nuisances remained the strongest predictors of outcomes.