The increase in urban heat waves has prompted cities to develop adaptation strategies. In 2018, the city of Paris launched a program to green schoolyards in order to reduce the heat island effect. Combining a difference-in-difference strategy and hedonic regressions, we assess the causal effect of green schoolyards on housing prices. The results show a significant increase in the price of dwellings located within 20m of a green school : three years after the renovation date, prices in these dwellings increase by 14\% compared to dwellings located close to a non-green school. We also find that the externality effect of schoolyard renovation declines steeply with distance. Our results therefore suggest that the greening of existing urban elements can improve living conditions in the surrounding area.