Basel Green Roof Policy

Basel, in northern Switzerland, is known for one of the world's most extensive green roof programs. It combined early subsidies with a legal requirement and is notable for its focus on biodiversity rather than stormwater alone.

Incentive programs

Basel ran two two-year subsidy programs, funded by an Energy Saving Fund that takes 5 percent of customers' energy bills:

• 1996-1997: subsidies of 20 Swiss francs per m² for new and retrofitted green roofs;
• 2005-2007: subsidies of 30-40 Swiss francs per m² for retrofits only.

Legal requirement

A 2002 amendment to the canton's Building and Construction Law required all new and renovated flat roofs to be greened according to design guidelines. A 2010 regulation extended this to all flat roofs in building retrofits and all new buildings with flat roofs.

Biodiversity rules

Basel's guidelines aim to recreate local habitats:

• substrate of native regional soil, at least 10 cm (4 in) deep, raised to 12 cm (about 5 in) in a 2015 amendment;
• a native plant mix characteristic of the Basel region;
• mounds about 30 cm (12 in) high and 3 m (10 ft) wide to create varied habitat for invertebrates;
• consultation with the city's green roof expert for flat roofs over 1,000 m² (about 10,800 sq ft).

Results

By 2006, about 23 percent of Basel's flat roof area was green, and around 100 roofs (80,000 m²) have been added each year since. By 2019, Basel had about 5.7 m² of green roof per inhabitant, described by the European Environment Agency's Climate-ADAPT platform as the largest in the world. The city estimates coverage of roughly 40 percent of flat roofs, though no official figure is available. Studies of insects and snails on the roofs were ongoing at the time of the Climate-ADAPT case study.

Influence

Basel's approach of pairing subsidies with mandates, and specifying native soils and plants, is frequently cited as a model for cities seeking both climate adaptation and habitat benefits from roofs.