ADU rules in Blaine County

Unincorporated Blaine County allows a detached accessory dwelling unit as an accessory use, but only in the A-20, A-40, R-10, RR-40, R-5, R-2 1/2, R-2, R-1, R-.4, R-1/4 and RD zoning districts, and not in any designated overlay district except three narrow cases: the floodplain overlay by conditional use permit, CH (Community Housing) overlay property that is not part of a CH-PUD, and Mountain Overlay land inside a platted building envelope or categorically excluded under 9-21-4. Whether the unit is by right turns on lot size rather than on the zone: one ADU is a permitted use on lots of 2 acres to under 5 acres and on lots of 5 acres or more, needs a conditional use permit on lots of 1 acre to under 2 acres, and is prohibited outright on lots under 1 acre unless the lot is served by an Idaho DEQ regulated community water system or a municipal water system, wins South Central Public Health District approval, and obtains a conditional use permit. A second ADU on a lot of 5 acres or more is available only by conditional use permit. Detached units get one explicit break: the 75 percent of primary building size cap that 9-2-1 imposes on accessory structures expressly does not apply to accessory dwelling units detached from any other building containing an accessory use.

The county definition reaches internal and attached units directly: an accessory dwelling is 'a separate building or portion thereof or portion of a residential building' that may provide complete independent living facilities, judged by whether the area is functionally separate from the primary residence floor plan, has a kitchen, and has a full bathroom (9-2-1). A basement suite or in-house conversion therefore runs through exactly the same 9-3-11 gates as a backyard cottage: same district list, same lot-size thresholds and conditional use permit triggers, same 1,200 square foot cap, same extra parking space. Section 9-3-4 closes the side door by providing that no accessory building on a lot with an existing principal dwelling may be converted to residential use except in conformance with 9-3-11. One carve-out keeps small additions out of the ADU regime entirely: a building or portion of a building of 500 square feet or less with no kitchen and no more than a half bath may be treated as an accessory building rather than an accessory dwelling unit.

Section 9-3-11.B.1 caps every accessory dwelling unit at 1,200 square feet of floor area, counted as the total of all floor areas measured from the exterior face of the exterior walls, and separately allows up to another 1,200 square feet of ADU garage space. Mechanical equipment counts toward the cap unless it sits in the crawl space or the attached ADU garage, stairs are excluded, and where an attic truss system creates unusable space the measurement is taken from the outside of the interior wall. Units of 400 square feet or less are built to International Residential Code Appendix Q (Tiny Houses). There is no percent-of-primary-dwelling cap on the ADU itself; the only percentage test appears in 9-3-11.B.10, where an ADU shares an accessory building with other accessory uses and the non-ADU portion may not exceed 75 percent of the gross floor area of the primary residence. Height is not addressed in 9-3-11 at all, so the district standard governs: 35 feet in the A-20, A-40, R-10, RR-40, R-5, R-2 1/2, R-2, R-1, R-.4 and R-1/4 districts.

Nothing in 9-3-11, or anywhere else in the Blaine County Code, requires an owner to live on the property. Full-text searches of the whole code for 'owner occupancy', 'owner occupied', 'occupied by the owner', 'shall reside' and 'principal residence' turn up no ADU-related requirement; the only occupancy-style condition in the ADU section is physical, that the unit 'shall be located on the same lot as the principal residential dwelling unit' (9-3-11.B.6). Idaho's 2026 statewide ADU statute forbids owner-occupancy requirements, but it does not reach this entry: Idaho Code 67-6541(6) provides that 'The provisions of this section shall apply only to cities with a population greater than ten thousand (10,000)', so Blaine County's freedom from an owner-occupancy rule is a purely local choice rather than a state floor, and the county could add one without violating state law.

Section 9-3-11.B.7 requires 'one parking space in addition to the minimum required for the principal residential dwelling unit', and B.8 adds that all parking for an ADU must be contained on-site with no street parking allowed. Because 9-28-2.B.1.a requires two spaces for a single-family dwelling, an ADU lot needs three off-street spaces in total. Each space must be at least 180 square feet, unobstructed, kept clear of snow, and surfaced with asphalt, concrete, compacted gravel or another dustless material (9-28-2.A). Idaho Code 67-6541(2)(b), which bars requiring ADU off-street parking in most situations, applies only to cities over 10,000 population and so does not disturb this county requirement.

The review track is set by lot size in 9-3-11.A. One ADU is a permitted use needing only a building permit (9-3-11.B.5) on lots of 2 acres to under 5 acres, and as the first unit on lots of 5 acres or more. A conditional use permit from the Planning and Zoning Commission or the hearing examiner is required on lots of 1 acre to under 2 acres, on lots under 1 acre (which additionally need an Idaho DEQ regulated community or municipal water system, a will-serve letter from the certified water operator, and South Central Public Health District approval), for a second ADU on lots of 5 acres or more, and for any ADU proposed in the floodplain overlay district. Conditional use permits follow the chapter 25 notice and hearing procedure, including mailed notice to owners within 300 feet, and are judged against the ten standards of evaluation in 9-25-3.A, with 9-3-11 pointing specifically at subsections A.3 (harmonious and appropriate in appearance with the character of the vicinity) and A.4 (not hazardous or disturbing to neighboring uses). A tiny home on wheels used as an ADU takes a zoning review and a Tiny Home on Wheels permit rather than a building permit, and that zoning approval terminates after 5 years subject to renewal.

Blaine County's code places no restriction on renting an accessory dwelling unit. Section 9-3-11 says nothing about tenancy length or tenant eligibility, and full-text searches of the entire county code for 'short term rental', 'short-term rental', 'vacation rental', 'nightly rental' and 'bed and breakfast' return zero hits, so there is no county short-term-rental licensing scheme, cap or permit to satisfy. State law now also forecloses creating one: Idaho Code 67-6539, rewritten by H0583 (2026 ch. 22, sec. 1, effective July 1, 2026), binds counties as well as cities. Subsection (1) bars any ordinance with the express or practical effect of prohibiting any type of short-term rental and classifies short-term rentals as a non-transient residential use; subsection (2)(a) lists owner-occupation requirements, day limits, extra parking, conditional use permits in residential zones, proximity spacing and numeric caps among the forbidden 'different restrictions or obligations'; and subsection (5) provides that 'No county or city shall require a license, fee, permit, certification, or registration to operate a short-term rental.' Generally applicable noise, parking, nuisance, curfew and traffic rules still apply under 67-6539(6). One local timing wrinkle: if the ADU is a tiny home on wheels, its zoning approval terminates after 5 years and must be renewed.