Boise, ID Accessory Structures: ADU Permits (2026)

Boise's modernized Zoning Code Title 11 (effective December 2023) and Idaho Code §67-6511A together establish one of the most permissive ADU frameworks in the western United States. Idaho Code §67-6511A, enacted by the 2023 Idaho Legislature, requires every city to permit at least one ADU on each single-family residential lot and prohibits owner-occupancy mandates that would make ADU construction impractical. Boise's Title 11 Chapter 11-04 implements this with: (1) ADUs permitted by right (no conditional use permit) in R-1A through R-3 districts; (2) maximum size of 900 sq ft or 10 percent of lot area; (3) detached, attached, or internal (basement, attic, interior conversion) configurations allowed; (4) maximum height generally matches the principal dwelling; (5) setbacks of 4 ft side and rear for detached ADUs under 15 ft tall; (6) one additional parking space encouraged but not required, particularly in transit-served areas. Permits required: zoning verification and building permit through Planning and Development Services (PDS), with the permit application available online. The 2018 International Residential Code (IRC) and 2017 NEC with Idaho amendments apply. Historic Preservation Commission (HPC) review is required for ADUs in designated historic districts (North End, East End, Warm Springs, Hyde Park). ADUs in the Foothills Hillside Development Overlay are subject to additional review. Stand-alone ADUs do not require separate utility meters; shared utilities with the principal dwelling are permitted.