Ada County, ID Environmental Rules: Stormwater Management (2026)

Section 8-3-5(B), Infrastructure: Drainage, requires that "all required drainage studies and drainage plans shall be based upon peak and total discharge estimates" and be prepared and submitted by a professional engineer licensed in Idaho. The drainage study itself must map the site's existing drainage and irrigation features, including ditches, watercourses and drainage structures; document soil permeability and geologic materials; size the upstream drainage basin; note site topography and slope; flag any surface water or potable water supply well within 100 feet; and describe the proposed use's potential contaminants along with any past soil or groundwater contamination.

Once the County Engineer requires a formal drainage plan as a condition of approval, that plan must be drawn to scale showing all proposed grading, the essential elements of the drainage system such as storm drains, catch basins, pumps and seepage beds, and flow arrows or contours showing exactly where stormwater will be routed on and off the property, including flows entering from adjoining land. Where drainage courses or irrigation facilities already exist on site, most notably the irrigation-district laterals and ditches running through many suburban Ada County subdivisions, the applicant must produce documentary evidence that every affected landowner and irrigation or drainage entity has consented to any re-routing before the County Engineer will approve the plan.

The same infrastructure chapter lets the County Engineer credit green stormwater infrastructure, rainwater harvesting or pervious pavement toward reducing required drainage improvements, but only if the applicant documents the reduction with accepted engineering methods and commits to a maintenance plan. Commercial or industrial buildings over 10,000 square feet, and single-family homes over 25,000 square feet, must also include working rainwater harvesting such as cisterns or rain barrels storing at least 40 gallons.