Boise, ID Accessory Structures: ADU Rules (2026)
ADU rules in Boise, ID, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.
Key Facts
- 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
Summary
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.
ADUs are permitted only as an accessory use to a single-family dwelling (attached or detached). A lot or parcel may contain up to two accessory residential units in any combination of ADUs or Tiny Homes on Wheels (THOW). Maximum density standards shall not apply to ADUs. The ADU shall not be larger than 900 square feet or 70 percent of the total floor area of the primary building, whichever is smaller, and shall not have more than two bedrooms. The ADU shall not exceed two stories in height.
Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: 2026 S-31: Code current through: Ord. 20-26, passed 6-9-2026).
Full Breakdown
Boise's Development Code allows ADUs only as an accessory use to a single-family home, attached or detached. A lot can contain up to two accessory residential units combining ADUs and Tiny Homes on Wheels, and maximum density standards don't apply to them. Each ADU is capped at 900 square feet or 70 percent of the primary structure's floor area, whichever is smaller, with no more than two bedrooms and a two-story height limit. Owners can create one by converting existing living space, adding onto the house or a detached accessory building, building a new detached unit, or converting a garage without cutting required parking below the minimum. Design must be compatible with the surrounding neighborhood.
Violations & Fines
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.
Frequently Asked Questions
Can I build a detached ADU in Boise?
Do ADUs count against Boise's density limits?
Sources & Official References
Other rules in Boise
How Boise compares: Easiest Cities to Build an ADU·Compare Boise to another location·View the Idaho accessory structures overview
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