5 county-level rules, plus city-specific rules for 1 city in Ada County, Idaho.
Verified from official government sources
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.
Ada County Code 8-4-23(C)(1)
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.
In unincorporated Ada County, a detached accessory structure (shed, garage, workshop) may not exceed 24 feet in height, cannot sit in a required setback or on an easement, and cannot be used as a dwelling. Small sheds 200 sq ft or less may sit in side or rear required yards.
Ada County Code 8-4-2(B)(3)
Height Standards. An accessory structure shall not exceed a height of twenty-four feet (24').
Unincorporated Ada County lets you convert an existing garage, basement, or detached accessory structure into a secondary dwelling, provided it meets the Section 8-4-23 standards, the Ada County Building Code, plumbing and electrical codes, and off-street parking for both units.
Ada County Code 8-4-23(E)
An existing garage, basement or a detached accessory structure may be converted into a secondary dwelling, provided that the conversion or structure meets the standards of this Section, along with all applicable requirements of the Ada County Building Code.
A carport is treated as an accessory structure in unincorporated Ada County. It cannot exceed 24 feet in height, cannot sit in a required setback or on an easement, and its roofing and finish materials must complement the principal home.
Ada County Code 8-4-2(B)(2)(a)
Accessory structures shall not be located in any required setbacks or on any publicly dedicated easements.
Ada County's zoning code treats a tiny home as a manufactured home. A tiny home used as a second unit must meet the secondary-dwelling standards of Section 8-4-23 (350 to 1,200 sq ft, foundation, deed restriction). A tiny home on wheels is a recreational vehicle, not a permanent dwelling.
Ada County Code Title 8, Ch. 10 (Manufactured Home)
Manufactured Home: A structure, transportable in one or more sections, built on a permanent chassis and designed for use as a dwelling... This includes Tiny Homes and Park Models. The term 'manufactured home' does not include a 'recreational vehicle.'
1 cities in Ada County have their own accessory structures rules. Each link goes to that city's dedicated page with code citations.
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