Rainwater harvesting for non-potable residential use (garden irrigation, lawn watering, toilet flushing with treatment) is legal in San Mateo County and encouraged by local water districts. Small rain barrels and above-ground cisterns up to 5,000 gallons generally do not require a permit. Larger systems, connections to plumbing, or potable-use systems trigger plumbing and building permits. Cal Water, BAWSD, Coastside County Water District, and other local agencies offer rain barrel rebates ($100-$300).
California AB 1750 (2012) and Health & Safety Code §116555 legalized residential rainwater capture for non-potable uses, and the 2019 CPC Chapter 16 and 17 address catchment and reuse plumbing. In unincorporated San Mateo County, rain barrels and cisterns used solely for outdoor irrigation do not require a permit if: (1) the system is above-ground, (2) total storage is under 5,000 gallons on a single parcel, (3) the system is not connected to plumbing, and (4) the installation includes first-flush diversion and mosquito-proof screening. Larger systems, underground cisterns, or any connection to building plumbing (for toilet flushing, for example) require a building and plumbing permit per 2022 CPC Chapter 16 (non-potable) or Chapter 17 (potable rainwater). Potable-use rainwater requires NSF-certified filtration and disinfection meeting Title 22 water quality standards. HOAs may restrict visible rain barrels. San Mateo County's urban water agencies — Cal Water (Peninsula, Bear Gulch), BAWSCA, Coastside County Water District, Westborough Water District — have historically offered $100-$300 rebates for rain barrel and cistern installations. Bay Area Greywater also allows simple laundry-to-landscape systems without permit under CPC §1502.1.1.
Unpermitted plumbing connection of cistern to house: building code violation $100-$500 plus required permit and inspections. Cross-connection with potable supply: Cal Water or district emergency shutoff and significant penalty under DDW rules. HOA violations: private association fines.
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