It is compulsory to have a rainwater harvesting system for all buildings measuring 1200 square feet and above. For earlier building built before 2009, it is compulsory if the area is above 2400 sq. feet. It could be either a storage system for re-use or used for groundwater recharge.
The regulations is avilable at https://www.bwssb.gov.in/document/home2/RWHGAZATTENOTIFICATIONS.pdf
Land based recharge should be done using groundwater recharge structures or pits when applicable. The capacity should be 10 litres or more per sq. meters of the land.
Rainwater will be harvested from roof of the buildings through pipelines into a storage tank or for recharge of an open well. Recharge pits should be there in case there are borewells instead of open wells. Appropriate filtering should be used and water should be treated to IS 10500 standards before using. A provision at the rate of 20 litres or more per sq, mtrs of the roof area should be adopted.
Recharge refers to the process of enhancing ground-water reserves by letting the collected rainwater sink into the ground. Appropriate filtering mechanisms, and pits with sufficient inflow capacity are prepared for this purpose. Benefits are sometimes not seen immediately, but long term benefits of doind recharge is proven beyond doubt.
Reuse refers to direct filtration and use of rainwater, by storing it in a sump/tank. As the water is directly consumed, benefits are immediate and evident.