Which is the first state to make rooftop rainwater harvesting compulsory?

Tamil Nadu is the first and the only state in India, which has made rooftop rainwater harvesting compulsory. Amendments made to Section 215 (a) of the Tamil Nadu District Municipalities Act, 1920 and Building Rules 1973, have made it mandatory to provide Rain Water Harvesting structures in all new buildings. 

Further Reading:

• Geography Notes
• Water – A Resource