No different than requiring proportions of parking spaces:
here’s one way

to push the solar deployment curve up faster
and hasten the year that most of the world’s power comes from the sun.

Agence France-Press 19 March 2015,

France decrees new rooftops must be covered in plants or solar panels,

Rooftops on new buildings built in commercial zones in France must
either be partially covered in plants or solar panels, under a law
approved on Thursday.

Green roofs have an isolating effect, helping reduce the amount of
energy needed to heat a building in winter and cool it in summer.
The argument for divesting from fossil fuels is becoming
overwhelming Read more

They also retain rainwater, thus helping reduce problems with
runoff, while favouring biodiversity and giving birds a place to
nest in the urban jungle, ecologists say.

-jsq