Satellite assessment of rooftop greenness dynamics and equity in Hong Kong

• High-resolution satellite imagery framework assesses rooftop greenness dynamics. • A new building-level index evaluates inequality in rooftop greenness. • Citywide greenness rose 22.1% while inequality declined across 452 communities. • Public housing showed stronger gains (38.9%) than private housing (21.1%) • Economic stress, non-local and single-person households linked to disparities. Rooftop greening is increasingly recognized as a pathway to urban sustainability and environmental equity, yet its fine-scale temporal dynamics and distributional fairness remain insufficiently examined. This study develops a building-level framework that constructs annual rooftop greenness time series from satellite-derived NDVI and introduces a Rooftop Greenness Inequality Index (RGII) based on the rooftop green fraction to evaluate distributive fairness. By integrating these measures with a multi-pattern trend classification, the framework captures the co-evolution of temporal dynamics and spatial inequality, and links them to socioeconomic conditions to identify key drivers and vulnerable communities. Using Hong Kong as a case study (2018–2024), we find overall greenness gains and inequality reductions, though unevenly distributed. Citywide, 22.1% of buildings showed significant increases, while 2.1% showed consistent decreases. All 452 communities saw inequality decrease, with the median RGII falling from 0.65 to 0.45. Greenness trajectories varied by land use and development stage: public housing showed a 38.9% rise compared with 21.1% in private housing, and late-stage new towns grew faster but with less stability than older districts. Socioeconomic disparities shaped outcomes: communities with high rent burden, large non-local populations, or many single-person households showed limited improvement, whereas those with more spatial capacity or stronger cultural cohesion performed better. Vulnerable areas were concentrated in older high-density districts where greenness remained low and inequality persisted. These findings demonstrate the framework’s capacity to integrate rooftop greenness change with social dimensions, providing transferable tools for equity-oriented greening strategies in high-density cities.