Mililani Town residents can collect rainwater for outdoor use without a permit up to 360 gallons; HOA rules may affect visible barrel placement.
Mililani Town's moderate Central Oahu rainfall supports rain barrel use in Mililani Mauka and older Mililani tracts, especially for vegetable gardens and fruit trees. Neither Honolulu ROH nor Hawaii state law restricts residential rainwater collection. Under the Hawaii Plumbing Code (2018 UPC Chapter 16), exterior catchment systems used for outdoor irrigation with storage up to 360 gallons require no permit. Larger systems or indoor tie-ins need plumbing permits and backflow protection. The Honolulu Board of Water Supply offers rain barrel rebates. Mililani Town Association CC&Rs may require barrels to be screened from street view or placed in side yards, so check architectural guidelines before installation.
Unpermitted plumbing connections or cross-connections to potable supply can trigger BWS enforcement; HOA violations are handled privately by the association.
See how other cities in Honolulu County handle rainwater harvesting.
See how Mililani Town's rainwater harvesting rules stack up against other locations.
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