Ada County, ID Landscaping Rules: Rainwater Harvesting (2026)
Key Facts
- Trigger size, commercial
- Over 10,000 sq ft requires harvesting
- Trigger size, residential
- Over 25,000 sq ft single-family home
- Minimum storage
- 40 gallons per cistern or rain barrel
- Effective date
- October 28, 2025 (Ord. 1002)
- Exempt uses
- Agricultural uses and pre-2025 development
- Approved water uses
- Irrigation, toilet flushing, pressure washing
Summary
Ada County Code 8-3-5(E)(4)(a) requires rainwater harvesting on any commercial or industrial building over 10,000 square feet and any single-family home over 25,000 square feet. Cisterns or rain barrels must hold at least 40 gallons, and the collected water can irrigate landscaping, flush toilets, or run pressure washers. The rule sits inside the county's infrastructure standards for new development.
4. Water Conservation Measures: a. Rainwater Harvesting. Rainwater harvesting includes, but is not limited to, cisterns and rain barrels. (1) All commercial and industrial structures over ten thousand (10,000) square feet require the use of rainwater harvesting techniques. (2) All single-family residences greater than twenty-five thousand (25,000) square feet require the use of rainwater harvesting techniques. (3) Cisterns and rain barrel design shall store at least forty (40) gallons of water. (4) Harvested rainwater from cisterns and rain barrels may be used for irrigating landscaped areas, flushing toilets, or pressure washing.
Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: 2026 S-15: Code current through: Ord. 1018, passed 3-24-2026).
Full Breakdown
Ada County Code 8-3-5(E) sets water conservation standards for new development in the unincorporated county, adopted as part of the Title 8 rewrite effective October 28, 2025 under Ordinance 1002. The rainwater harvesting mandate at subsection (E)(4)(a) applies once a building crosses a size threshold: commercial and industrial structures over 10,000 square feet and single-family residences over 25,000 square feet must incorporate cisterns, rain barrels, or comparable harvesting techniques. Each cistern or rain barrel has to be sized to hold at least 40 gallons, and the code spells out permitted uses for the collected water: irrigating landscaped areas, flushing toilets, or pressure washing.
The requirement is one of four numbered water conservation measures in subsection (E)(4), grouped alongside pervious pavement standards at (E)(4)(b), which allow porous asphalt, pervious concrete, and permeable pavers for parking surfaces and let the County Engineer credit that permeability against drainage study requirements. Subsection (E)(3) exempts all agricultural uses and any use that existed before the Title 8 rewrite took effect, so an established farm or a pre-2025 home under the 25,000-square-foot threshold is not retrofitted into compliance. The County Engineer also administers a broader green infrastructure option under (E)(5): a developer can substitute green stormwater infrastructure, added rainwater harvesting, or pervious pavement for some drainage improvements if the application shows the measures will cut runoff using accepted engineering principles and the applicant files a maintenance plan for the facilities.
Because the rainwater harvesting duty is triggered by building size rather than district or use type, it reaches large accessory structures and shops built on rural residential lots as readily as new commercial buildings, and compliance is verified through the master site plan and building permit review the Director of Development Services conducts.
Violations & Fines
Water conservation standards are part of the Zoning Ordinance, so violations are enforced under Ada County Code 8-8-2: a conviction is a misdemeanor punishable under Idaho Code 18-113, and each day noncompliance continues after notice counts as a separate offense. The Director of Development Services can also withhold sign-off on the required infrastructure improvements until a cistern or rain barrel is installed and inspected.
Frequently Asked Questions
Does my house in unincorporated Ada County need a rainwater harvesting system?
What size cistern or rain barrel satisfies the county's rule?
Are farms exempt from the rainwater harvesting requirement?
Sources & Official References
Other rules in Ada County
Compare Ada County to another location·View the Idaho landscaping rules overview
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