Equitable Implementation of Green Roofs in The Hague

Abstract

Cities are increasingly confronted with climate change impacts such as flooding and the urban heat island effect. Municipalities turn to nature-based solutions such as green roofs to improve resilience while generating environmental and social co-benefits. Yet implementation is not automatically equitable. Without explicit attention to who bears the costs and who receives the benefits, policies risk reinforcing existing socio-spatial inequalities, for instance through processes of green gentrification. Conventional cost-benefit analysis and even social cost-benefit analysis demonstrate that green roofs can provide positive returns in aggregate, but rarely reveal how outcomes differ across households, housing types, or neighbourhoods. This limits their usefulness for decisions that must balance both efficiency and fairness. The Hague provides a timely case: the city faces flood and heat challenges, has ambitious adaptation goals, and exhibits sharp variation in socio-economic status across its neighbourhoods.
The central question guiding this thesis is: How can social cost-benefit analysis and spatial equity analysis support policy making for equitable and effective implementation of green roofs in The Hague?
The research framework combines several steps. A literature review identified recognised costs and benefits of green roofs and highlighted persistent gaps in monetisation, especially for co-benefits such as biodiversity and mental health. A city-wide social cost-benefit analysis was then carried out using harmonised GIS-based data on roof suitability, climate vulnerability, and socio-economic conditions. Four expansion scenarios were analysed: environmental-priority, income-based, uniform, and optimised allocation. Results were evaluated through equity analysis, using indicators such as the Gini coefficient and Lorenz curves, followed by clustering into neighbourhood typologies. Representative neighbourhoods were decomposed into postcode-5 units for detailed analysis disaggregated by housing type (owner-occupied, private rental, social rental). Finally, robustness was tested with exploratory model analysis, including Latin Hypercube Sampling, tornado diagrams, and partial dependence diagnostics.
The current stock of green roofs in The Hague covers about 323,000 m² but delivers limited societal returns. All neighbourhoods record negative net present values, and no benefit–cost ratio exceeds one. Expansion scenarios substantially improve performance: uniform, income-based, and environmental allocations enable roughly two thirds of neighbourhoods to surpass break-even, showing that scaling up can create sufficient critical mass for societal benefits to outweigh costs. The optimised allocation, in contrast, delivers very high returns in a small number of neighbourhoods while leaving most of the city unchanged, achieving efficiency at the expense of coverage.
Distributional outcomes differ strongly by allocation rule. The income-based strategy performs best: it reduces inequality, directs benefits to lower-income and climate-vulnerable areas, and is the only scenario that achieves positive alignment between benefits and adaptation need. Uniform and environmental allocations increase coverage and improve averages, but fail to systematically prioritise vulnerable groups. The optimised allocation generates extreme inequality by concentrating benefits in affluent, low-density areas with high property values, while excluding most neighbourhoods.
Clustering reveals four neighbourhood types: High Gain–Equity, Low Uptake–Stable, High Risk–Reward, and Low Return–Saturated. Detailed analysis of representative cases shows how socio-economic context shapes outcomes. Affluent, low-density areas such as Zorgvliet incur the largest losses despite high roof potential, as costs are spread across fewer households and collective benefits remain limited. By contrast, dense, mixed-income areas such as Oostbroek-Zuid and Moerwijk-Noord perform relatively better in per-household terms, as health and energy benefits accumulate across many dwellings. Tenure also proves decisive: homeowners capture most property-related benefits, while tenants, particularly in social housing, gain relatively little, and housing corporations bear a disproportionate share of costs.
Uncertainty analysis confirms that results are sensitive to assumptions about installation costs, annual maintenance, flood damages, energy prices, and property values. While absolute figures shift under different assumptions, the relative ranking of scenarios remains broadly consistent. At the same time, outcome ranges overlap, meaning no allocation is universally superior. This highlights the importance of designing flexible policies and safeguards for vulnerable groups to ensure that equity goals are met under uncertainty.
Overall, the findings demonstrate that green roofs can contribute meaningfully to The Hague’s climate adaptation goals, but distributive outcomes vary strongly by allocation strategy, neighbourhood type, and tenure composition. Efficiency and fairness are not automatic by-products of expansion; they must be deliberately embedded in policy design. For policymakers, this means that targeting denser, lower-income areas can combine efficiency with equity, collaboration with housing corporations is essential to avoid overburdening social renters, and evaluations must acknowledge co-benefits that remain difficult to monetise. By integrating social cost-benefit analysis with spatial equity assessment, this thesis shows how economic evaluation can be extended beyond aggregate efficiency to incorporate distributive, procedural, and recognitional justice. For The Hague and similar mid-sized European cities, this provides a framework for implementing green roofs not only as effective climate measures but also as pathways towards more inclusive and just urban futures.