Ada County, ID Accessory Structures: ADU Rules (2026)
ADU rules in Ada County, ID, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.
Key Facts
- Units allowed
- One per lot
- Minimum lot size
- 6,000 square feet
- Size range
- 350 to 1,200 sq ft
- Max bedrooms
- Three
- Code section
- Ada County 8-4-23
Summary
Unincorporated Ada County allows one secondary (accessory) dwelling per lot of at least 6,000 square feet. It must be 350 to 1,200 square feet, or 60% of the principal home's above-ground living space, whichever is less, and capped at three bedrooms.
A secondary dwelling may be between three hundred fifty (350) square feet up to a maximum of one thousand two hundred (1,200) square feet or sixty percent (60%) of the square footage of above-ground living space of the principal dwelling, whichever is less.
a. Sixty Percent (60%) Calculation: The square footage of covered outdoor space, basements or attached/detached garages associated with the principal dwelling must be excluded from the sixty percent (60%) calculation.
2. Bedrooms: Secondary dwellings are limited to a maximum of three (3) bedrooms.
3. Parking: Off-street parking shall be provided as per Section 8-3-10 in addition to the required off street parking for the principal dwelling.
4. Appearance: The secondary dwelling shall be complementary with the principal permitted dwelling in that it:
a. Does not appear like a second principal dwelling;
b. Has a similar roof pitch; and
c. Has similar color of building materials.
5. Facilities: The secondary dwelling shall have separate kitchen and bathroom facilities from the principal permitted dwelling.
6. Utilities: The secondary dwelling shall be provided with utilities which may be metered separately - including gas, electric, communications, water and sewer all subject to the approval of applicable utility companies and public agencies.
7. Entry: Secondary dwellings shall be provided with a primary point of
Full Breakdown
Ada County Code Title 8 (Zoning), Section 8-4-23 governs secondary attached or detached dwellings on unincorporated land. A principal dwelling with a certificate of occupancy must already exist. Only one additional dwelling is allowed per property, on a lot of at least 6,000 square feet, and the owner must occupy either the principal or the secondary dwelling. The unit must be 350 to 1,200 square feet (or 60% of the principal home's above-ground living space, whichever is less), have no more than three bedrooms, separate kitchen and bath, its own entrance and address, and sit on a foundation. A recorded deed restriction is required. Inside Boise, Meridian, Eagle, Kuna, or Star, the city's ADU rules apply.
Violations & Fines
Building or renting a secondary dwelling without meeting Section 8-4-23 (size, occupancy, deed restriction) is a zoning violation enforced by Ada County Development Services.
Frequently Asked Questions
How big can an ADU be in unincorporated Ada County?
Can I rent out my secondary dwelling?
Sources & Official References
Other rules in Ada County
How Ada County compares: Easiest Cities to Build an ADU·Compare Ada County to another location·View the Idaho accessory structures overview
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