Side by side · Idaho
Compare ADU rules across Idaho
Flexible on where you build? The same backyard cottage can be routine in one city and impossible next door. This table puts 14 Idaho cities side by side so you can shortlist the friendly ones before reading a single ordinance, or narrow it to just the cities you are deciding between. Every value is summarized from the city's own code; open a city's page for the full notes, citations, and fine print.
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| City | Detached ADU | Attached / internal | Max size | Owner occupancy | Extra parking | Approval |
|---|---|---|---|---|---|---|
| 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. Ordinance read August 2026 | 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. Blaine County Code 9-2-1 (definition of Dwelling, Accessory); 9-3-4; 9-3-11Full rules for Blaine County Ordinance read August 2026 | 1,200 sq ft of floor area, plus up to 1,200 sq ft of ADU garage; height set by the district (35 ft in the residential and agricultural zones) | Not required | 1 space in addition to the primary dwelling's requirement, all on-site (3 spaces total for a single-family lot) | 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 Code 9-3-11(A)(1) to (A)(4), (B)(3), (B)(5), (C)(6), (C)(7); 9-25-3(A)Full rules for Blaine County Ordinance read August 2026 |
| Boise | Allowed | Allowed | 900 sq ft or 70% of primary dwelling floor area (whichever is smaller); max 2 bedrooms; max 2 stories | Not required | None | Permitted use |
| Caldwell | Allowed | Allowed | 900 sq ft or 75% of the principal residence, whichever is less; 2 bedrooms max | Not required | None: minimum 0 spaces (maximum 1 allowed) | Permitted use |
| Coeur D’Alene | Allowed | Allowed | 800 sq ft max (finished area) and no more than 75% of the primary dwelling excluding garage; no minimum; height 18 ft to 32 ft by placement | Not required | 1 additional off-street space today; state law forces removal of the requirement by 2-1-2027 outside narrow carve-outs | Permitted use |
| Idaho Falls | Allowed | Allowed | 750 sq ft today; rises to 1,000 sq ft or 75% of the primary dwelling, whichever is greater, on February 1, 2027 | Required | 1 space for the ADU on top of the house's own requirement (2 for a single-unit detached home); drops to none on February 1, 2027 | Permitted use |
| Ketchum | Allowed | Allowed | 1,200 sq ft of net livable space (300 sq ft minimum) | Not required | None | Permitted use |
| Lewiston | Allowed | Allowed | 1,000 sq ft or 75% of the primary dwelling's habitable floor area, whichever is greater, with no absolute ceiling and height capped at the house's height (Ord. No. 4972, 2026). The text still codified on Municode allows only 600 sq ft or 30%, and never more than 900 sq ft. | Not required | 1 space per bedroom, up to 2, but none at all if the house has no off-street parking, the street is unpaved, or the house is within 1/4 mile of transit, an employment area or commercial services (Ord. No. 4972, 2026). The still-codified text asks for 1 space, or 1 per bedroom in the Normal Hill zones. | Permitted use |
| McCall | Allowed | Allowed | 1,500 sq ft (no size limit if deed restricted as a local housing unit) | Not required | None | Permitted use |
| Meridian | Allowed | Allowed | 900 sq ft or 50% of the primary dwelling, whichever is less; maximum 2 bedrooms | Not required | Yes: the whole property is re-rated on the single-family schedule by combined bedroom count (2 spaces at 1-2 bedrooms, 4 at 3-4, 6 at 5+), so an ADU often adds 2 required spaces | Permitted use |
| Moscow | Allowed | Allowed | 600 sq ft, or 40% of the principal dwelling, whichever is less; 2-person occupancy cap | Required | 1 off-street space, in addition to the principal dwelling's | Permitted use |
| Nampa | Allowed | Nampa recognizes only a DETACHED accessory dwelling. Title 10 contains no attached or internal ADU use anywhere, the 10-1-2.H definition is written for 'a separate structure', and the Planning and Zoning FAQ tells homeowners an ADU 'is detached from the primary residential dwelling unit' while an attached unit is either an addition (accessible to and from the main home, so not a separate dwelling) or a duplex. A genuinely separate attached or basement unit therefore reads as a two-family dwelling, which 10-3-2 allows only by conditional use permit in the RS single-family zone and does not allow at all in the AG or RA zones; duplexes are outright permitted in RD, RML, RMH and RP. Idaho Code 67-6541(2)(a) is written in the alternative, requiring covered cities to allow 'either one (1) internal accessory dwelling unit within the single-family dwelling or one (1) detached accessory dwelling unit per lot', so a detached-only city can satisfy the state floor without ever adding an internal option. Nampa City Code § 10-1-2.H, Accessory dwelling (residential), detached; § 10-3-2 ('Dwelling, duplex (two-family)' = C in RS)Full rules for Nampa Ordinance read August 2026 | 500 sq ft of livable space, one bedroom only | Not required | No ADU-specific rule, but RS zone text reads 2 spaces per dwelling unit | Permitted use |
| Pocatello | Allowed | Allowed | 750 sq ft, and no more than 1 bedroom; height capped at the main house or the district maximum, whichever is less | Required | 1 additional off-street space if on-street parking is available beside the house; 2 spaces if it is not | Permitted use |
| Post Falls | Allowed | Allowed | 1,000 sq ft or 50% of the primary residence's living area, whichever is smaller | Required | 1 additional paved off-street space | Permitted use |
| Twin Falls | Allowed | Allowed | 1,000 sq ft or 75% of the principal residence's living space, whichever is greater; height limited to the principal residence's base-district maximum (35 ft in TN-1 and TN-2, 40 ft in AG, RR and TN-3) | Not required | None in the normal case; 1 additional space only if the principal dwelling has no off-street parking, or the adjoining street provides no on-street parking | Permitted use |
Summarized from each city's ordinance text, most recently read August 2026. Tap a value marked with an info icon to see exactly what it depends on, straight from the ordinance. Nearly every value has zone, lot-size, or process caveats beyond that; the per-city pages carry the full notes and code citations. Treat this as orientation, not a permit determination.
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Reading the table
- Detached ADU is the make-or-break column for backyard cottages: minimum lot sizes (often 8,000 to 14,500 sq ft) hide behind many of the "Allowed" values, so check your lot before celebrating.
- Attached / internal (basement apartments, additions) is legal almost everywhere and is usually the path of least resistance.
- Approval: "Permitted use" means staff-level sign-off; "Conditional use" means a public hearing, which adds time, cost, and uncertainty.
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