Several states and communities have enacted regulations and guidelines for dealing with rainwater harvesting, including Tucson, Flagstaff, Chino Valley, and Pason (in Arizona); Seattle and Friday Harbor, as well as King County (Washington state); and Atlanta, Georgia.
The American Rainwater Catchment Systems Association is working with several national organizations to come up with general ordinances that communities can revise and adopt. Texas has state legislation dealing with rainwater harvesting and, along with Hawaii and Virginia, offers rainwater harvesting guideline booklets (see Access). The Uniform Plumbing Code has an appendix dedicated to rainwater systems, with sizing guidelines for gutters, downspouts, and lateral pipes.
Beware, though, not every locale is rainwater-system friendly, so if you want to keep to the letter of the law, you’ll need to check with your local authority having jurisdiction to see what, if any, rainwater harvesting regulations are in place.