governs every rainwater-harvesting setup in the United States. Requirements shift depending on where you live and what you are actually doing.
A few things can change the answer: the source of the water, the type of collection system, how you store it, and what you plan to use it for. Rooftop rainwater harvesting is one category. Capturing or obstructing naturally flowing surface water is a very different one. The rules can be stricter for the second.
Depending on your system, plumbing codes, health rules, building requirements, or water-rights law might all apply. A small barrel catching gutter runoff sits at one end of that range. A pond or reservoir sits at the other. That is why the honest guidance is to check current state and local requirements. Do this rather than trust a blanket claim that it is legal or illegal everywhere. Both versions of that sweeping statement are wrong.
