Idaho ADU permit requirements
Last reviewed: June 20, 2026
Quick answer
Permitting an accessory dwelling unit (ADU) in Idaho means pulling a building permit from the local authority that issues it. Idaho has a statewide ADU law that sets baseline standards local agencies must follow. ArchPermit tracks 13 source-cited requirements (11 required, the rest conditional) for this jurisdiction. These requirements were last reviewed June 20, 2026; confirm the exact forms and fees with the local permitting authority.
Idaho has a statewide ADU law. This baseline reflects that law plus the universal new-dwelling deliverables. Your local city or county still administers the permit and may add its own forms and fees, so confirm the exact checklist with them.
Idaho's SB 1354 (2025, effective July 1, 2026) requires cities to allow one ADU per single-family lot (internal/attached OR detached); HB 166 (2023) already barred outright bans on attached ADUs. The local city/county administers permits and sets detailed standards; confirm locally.
- Adopted building code compliancerequired
The ADU must comply with Idaho's adopted residential building code: 2018 IRC (2020 Idaho Residential Code). Confirm the in-force edition with the local AHJ.
source · reviewed 2026-06-20 - Building cross-sectionsrequired
Cross-sections through the structure showing ceiling heights, wall/roof assemblies, insulation, and fire/sound-rated assemblies where required.
source · reviewed 2026-06-20 - Egress & life-safetyrequired
Emergency escape and rescue openings (egress windows), smoke and carbon-monoxide alarms, and required exits per the adopted residential code.
source · reviewed 2026-06-20 - Energy code compliancerequired
Energy-code compliance documentation for the conditioned space. Idaho: 2018 IECC (with Idaho amendments).
source · reviewed 2026-06-20 - Exterior elevationsrequired
Elevations of each affected building face showing height, exterior materials, and openings; existing vs. proposed where exterior changes occur.
source · reviewed 2026-06-20 - Floor plansrequired
Dimensioned floor plans of the ADU (and affected areas of the primary dwelling) showing rooms, uses, door/window locations, egress, and fixture/equipment locations.
source · reviewed 2026-06-20 - Foundation planrequired
Foundation plan with footing/stem-wall sizes and details (or a cited site-specific design), per the adopted residential code.
source · reviewed 2026-06-20 - Framing / structural plansrequired
Floor, wall, and roof framing showing member sizes, spans, spacing, connections, and lateral bracing; structural calculations where the framing is non-conventional.
source · reviewed 2026-06-20 - Local zoning / setback compliancerequiredverify locally
Compliance with the local jurisdiction's zoning: permitted use, setbacks, height, lot coverage, and any ADU-specific standards. These are set locally. Confirm with the permitting agency.
source · reviewed 2026-06-20 - Mechanical / plumbing / electricalrequired
HVAC, plumbing fixtures and lines, and electrical (panels, circuits, outlets, lighting, smoke/CO alarms) for the new unit.
source · reviewed 2026-06-20 - Site / plot plan (to scale)required
Scaled site plan showing property lines, setbacks, existing structures, the proposed ADU footprint with dimensions to property lines, driveways/parking, and utility locations.
source · reviewed 2026-06-20 - No outright ban on attached ADUsconditional
Jurisdictions cannot impose an outright ban on attached ADUs (HB 166, 2023).
source · reviewed 2026-06-20 - One ADU per single-family lot (internal OR detached)conditionalverify locally
Cities must allow one ADU per single-family lot, either an internal/attached unit or a detached rear-yard unit (SB 1354).
source · reviewed 2026-06-20
Idaho ADU permit questions
- Do I need a permit to build an ADU in Idaho?
- Yes. An accessory dwelling unit (ADU) in Idaho requires a building permit, and Idaho has a statewide ADU law that sets baseline standards local agencies must follow. ArchPermit tracks 13 cited requirements for this jurisdiction (11 required, the rest conditional). Always confirm the exact forms, fees, and code edition with the local permitting authority that issues the permit.
- What does an ADU permit package require in Idaho?
- Core required items include: Adopted building code compliance; Building cross-sections; Egress & life-safety; Energy code compliance; Exterior elevations; Floor plans, plus 5 more. Each requirement links to its authoritative source, and nothing is permit-ready until an independent licensed professional reviews and seals it.