Opportunities and constraints of implementing the 3–30–300 rule for urban greening

Urbanisation and climate change have increased the need for equitable access and visibility of urban green and blue spaces (GBS), to promote the sustainability and resilience of cities and to improve the well-being of their inhabitants. In this paper, we test an implementation of the newly proposed guideline to achieve equitable greening, the 3-30-300 rule, in three European cities: Paris Region (France), Aarhus Municipality (Denmark), and Grad Velika Gorica (Croatia). In this analysis, every residential building should have at least three viewable trees, 30% neighbourhood GBS cover, and a GBS of at least 1 hectare within 300 m. Our results show that none of the cities currently meet any of these three components, and the three cities differed in which rules were most closely met. In our implementation, substantial changes were needed in all cities to meet the guidelines: 12.6% of Paris, 10% of Aarhus, and 18.4% of Velika Gorica's urban footprint were converted to grass or tree cover, with implications for >100,000 buildings and >900,000 inhabitants. Our study discusses how existing conditions in each city impacted the viability of meeting the rule and proposes key considerations for future implementations of such guidelines, drawing on examples of innovative GBS already implemented globally.