Ada County, ID Fence Regulations: Retaining Walls (2026)

Section 8-2-30(E)(4)(a), the Hillside Overlay District's Grading and Stabilization standards, requires that "all retaining walls with a total vertical height of four feet (4') or more, including footing, shall be designed in accord with the regulations of the Ada County Building Code," pulling the wall into the same engineered-design and permit review that governs structural work under Title 7. The Hillside Overlay itself applies to any grading, filling, clearing or excavation on slopes of 25 percent or steeper, split into a Level 1 tier for ordinary steep-slope sites and a Level 2 tier for sites that also carry a documented special hazard such as an active landslide area, a fault line, expansive or hydro-compactible soils, or a water table within six feet of the surface.

A retaining wall built as part of that grading must also meet the chapter's broader stabilization rules: slopes stabilized mechanically or chemically must "conform to the surrounding terrain" with matching aesthetic treatment, all disturbed soil surfaces must be stabilized or covered before December 1 each year, with temporary erosion control installed if the finished surface is not ready by then, and construction is supposed to be scheduled to avoid soil disturbance between December 1 and April 15 altogether. Where a retaining wall sits near a watercourse, the same section bars depositing construction material such as soil in any floodway or watercourse and requires drainage structures below the wall to be riprapped or otherwise stabilized to prevent channel erosion.

A licensed professional engineer must certify the preliminary and final grading plans for Level 1 and Level 2 sites, and the County Engineer reviews and recommends approval, approval with conditions, or denial to the Director before any grading, including wall construction, can begin.