How Boise Handles Accessory Structures: A Practical Guide

Boise maintains 141 local ordinances across all categories, and 5 of those deal specifically with accessory structures. Here is a breakdown of what the city actually requires, what is prohibited, and where Boise falls on the strict-to-permissive spectrum compared to other cities.

Garage Conversions

Boise allows turning a garage into living space as an accessory dwelling unit, but only if replacement off-street parking is sited elsewhere on the lot and the unit stays under 900 square feet.

Key details: Size cap: 900 sq ft or 70% of home, smaller. Parking: Must replace parking lost on same lot. Bedrooms: Max two bedrooms per ADU. Height: Max two stories. Review: Zoning review plus building permit required.

Converting a garage without meeting ADU parking, size, and design standards or without a permit can trigger a stop-work order and code enforcement action from Boise's Planning and Development Services.

Tiny Homes

Boise allows a tiny home on wheels as an accessory unit to a single-family home, capped at 400 square feet and subject to permitted utility hookups.

Key details: Max size: 400 square feet. Allowed use: Accessory to single-family home only. Unit cap per lot: 2 total ADUs/THOWs combined. Power source: Generators prohibited. Certification: NOAH+ standard required before occupancy.

Occupying a THOW without a NOAH+ certification on file, exceeding two accessory residential units, running it on a generator, or renting it short-term can draw code enforcement action and required corrections before occupancy is allowed to continue.

Shed & Carport Rules

Detached carports in Boise can't sit in front of the house or encroach into front and street-side setbacks without a conditional use permit, though one-story carports under 200 square feet skip a building permit entirely.

Key details: Front-yard placement: Barred without conditional use permit. Setback rule: No encroachment into front/street-side setback. Alley-loaded carports: Under 1,000 sq ft may reach alley line. Building permit exemption: One-story, 200 sq ft or less. Enforcement: Misdemeanor, separate offense per day.

Zoning Code violations, including unpermitted setback encroachments, are misdemeanors under Boise City Code § 1-4-1; each day a violation continues counts as a separate offense enforced by Code Compliance.

ADU Rules

Boise allows up to two accessory dwelling units per single-family lot, capped at 900 square feet or 70% of the primary home's floor area, with no more than two bedrooms and two stories in height.

Key details: Max ADUs per lot: Up to 2 (ADU/THOW combined). Max size: 900 sq ft or 70% of home. Bedroom cap: 2 bedrooms max. Height limit: 2 stories max. Density exemption: ADUs exempt from density caps.

Building an ADU beyond the 900-square-foot/70-percent cap, adding a third bedroom, or skipping the required Building Permit invites a stop-work order and code enforcement action from Boise's Planning and Development Services until the unit is brought into compliance.

ADU Owner Occupancy

Inside the Harris Ranch Specific Plan area, Boise requires the principal home or its accessory dwelling to stay owner-occupied on an ongoing basis. The Planning Director cannot approve the accessory dwelling unless that commitment is recorded as a deed restriction with the Ada County Recorder before occupancy begins.

Key details: Applies to: Harris Ranch Specific Plan area only. Enforcement mechanism: Recorded deed restriction, Ada County Recorder. Waiver limit: Up to 1 year, hardship only. ADU footprint cap: 10% of lot or 750 sq ft, less of two. Bedroom limit: 1 bedroom maximum.

Violations of Title 11, including a breach of the recorded owner-occupancy deed restriction or building/occupying an accessory dwelling without the required Planning Director findings, are enforced under Boise City Code § 11-05-07. The Planning Director and Code Compliance Officers may investigate, issue notices of violation, and pursue injunctive relief. Any violation constitutes a misdemeanor under Boise City Code § 1-4-1, and a separate offense occurs for each day the violation continues.

This is not one of those rules that cities tend to ignore. Boise actively enforces its adu owner occupancy requirements.

The Bottom Line

Boise's accessory structures rules are a mixed bag. Some areas are strict, others are relaxed, and the details matter. The best approach is to check the specific rule that applies to your situation rather than assuming Boise is broadly strict or permissive.

All of the above reflects Boise's municipal code as of our last review. If you need specifics on fines, exemptions, or filing requirements, the detailed ordinance pages linked above have the full breakdown.