Idaho ADU
Building an ADU in Idaho — documented from Day 1.

The laws that govern an ADU in Idaho

By Russell Mann — Post Falls, Idaho · Updated July 13, 2026

This page is only useful if it's right. If you find a mistake, tell me and I'll fix it.

Three layers of law decide whether you can build an ADU on your Idaho lot: the state law, your city or county's code, and a set of rules that apply no matter where you are. This page indexes all three. Click your jurisdiction below and you get the specific set of rules that governs your property.

Two warnings before you click. First, these rules are changing right now: the state law took effect July 1, 2026, and covered cities have until February 1, 2027 to update their codes, so a city's published rules can disagree with state law today. Every fact here carries the date I verified it. Second, where a city code is stricter than the state law, that's a conflict in the text. Whether the state limits are enforceable before the city amends its code is an open legal question. I never claim a city rule is void, and neither should you without a lawyer.

Layer 1: the state law, Idaho Code § 67-6541 (SB 1354)

Signed March 31, 2026, effective July 1, 2026 (source, as of 2026-06-30). The full text is at legislature.idaho.gov; everything in this list comes from that statute text (verified 2026-07-03). What it does:

For a worked example of how this statute collides with one real city code, read my side-by-side of the state law and the Post Falls code.

Layer 2: your city or county's code

Where is your property? Pick one. And check before you assume: a city mailing address does not prove the lot is inside city limits. Confirm with your county or city GIS map, or a call to the planning department, before relying on anything below.

Key. Every jurisdiction gets tags so you can read its status without opening it:

⚠ conflict the local text conflicts with the state law (covered cities only)   ✓ clear a clear ADU path; for covered cities, no conflict identified in the text so far   ? open open questions I'm still tracking; click in to see them   ◑ partial my survey of this code is incomplete   – none no ADU provision found in the code

Inside an entry, the same colors sit on the specific points: ⚠ marks the rule that conflicts, ? marks the point that's still open.

Pick the jurisdiction whose zoning actually governs your lot. Inside city limits, that's the city, not the county. The state law section above and the everywhere rules below apply to you either way.

In a city over 10,000 people · state law applies · 25 cities

These 25 cities are covered by § 67-6541 (population estimates as of 2026-07-04, from idaho-demographics.com; two borderline cities are flagged). Each city's own code still governs everything the state law doesn't override, and most of these codes were written before the state law existed. Entries note where the local text conflicts with the statute.

Ammon · allowed in 4 zones, permit required ⚠ conflict
  • Code: Ammon City Code Title 10, Ch. 7 § 4, as summarized in the city's own handout.
  • One ADU per lot; attached, detached, or basement. RVs and travel trailers excluded. Permit through the Ammon Building Department.
  • Only allowed in 4 residential zones (RE, RP, RP-A, R-1). Not allowed in R-2 or R-3, the zones that otherwise permit duplexes and apartments.
  • ⚠ Vs. state law: the size rule picks the smaller of the two state numbers rather than the larger, a likely conflict.

Source: city ADU regulations handout, verified 2026-07-04.

Blackfoot · conditional use permit only, 2 zones ⚠ conflict
  • Blackfoot has no dedicated ADU standards section and no definition of "accessory dwelling unit" anywhere in its code (confirmed 2026-07-05).
  • ADUs are mentioned in only 2 of 8 residential zone descriptions (LDR2 and MDR2), both requiring a Conditional Use Permit judged against the code's general CUP criteria (§ 11-6A-4).
  • ⚠ Vs. state law: a discretionary CUP process conflicts with the statute's matter-of-right requirement.

Source: Blackfoot City Code §§ 11-4A-5, 11-6A-4, 11-1-3 (current through Ord. 2283), verified 2026-07-05.

Boise · by right, no parking or owner-occupancy rules ⚠ conflict
  • Code: Boise Development Code Title 11, Ch. 11-03 (the 2023 citywide rewrite, effective 2023-12-01).
  • One ADU per lot; internal conversion, addition, or detached; max 2 stories, max 2 bedrooms.
  • No additional off-street parking requirement and no owner-occupancy requirement. Both already match the state law.
  • ⚠ Vs. state law: size cap is 900 sq ft or 70% of the primary, whichever is smaller. That's below the state's 1,000 sq ft / 75% floor.

Source: Boise Development Code (current through Ord. 10-26), verified 2026-07-04.

Burley · allowed in R-1, one-sentence standard ? open
  • Code: Burley City Code § 5-5-1(A). ADUs are named in the R-1 district's permitted uses.
  • The entire operative text is one sentence with a setback rule (25 ft from the rear lot line for accessory buildings with sleeping quarters). No size cap, no parking rule, no owner-occupancy rule, no ADU definition found in the zoning definitions chapter.
  • ? Vs. state law: the code is too thin to compare. How the city handles a real application is an open question, flagged for a follow-up call.

Source: Burley City Code § 5-5-1, verified 2026-07-04.

Caldwell · by right, no owner-occupancy or parking requirement ✓ clear
  • Code: Caldwell City Code § 10-12-03-1(4).
  • Size: 900 sq ft or 75% of the principal residence, whichever is less. Nearly the state's own formula.
  • No owner-occupancy requirement and no ADU-specific parking requirement found anywhere in the section.
  • Caldwell also opened its historic district to ADUs, which the state law doesn't require. One of the most already-compliant cities surveyed.

Source: Caldwell City Code § 10-12-03-1, verified 2026-07-04.

Chubbuck · neighbor consent decides the permit path ⚠ conflict
  • Code: Chubbuck City Code Ch. 18.10 (Ord. 823, 2021; code current through Ord. 887, 2026-03-04).
  • In R-1/R-2/R-2P, the permit path depends on unanimous neighbor consent: signatures from every property owner within 300 feet along the street get an administrative permit; a single refusal forces a Conditional Use Permit. In A, C-2, and I zones a CUP is always required.
  • Owner-occupancy required (owner lives in one of the two units) plus a recorded deed restriction before the building permit issues.
  • Size: 1,000 sq ft cap for residential ADUs. One added parking space, waivable by the Director.
  • ⚠ Vs. state law: the neighbor-consent/CUP mechanism and the owner-occupancy rule both conflict with the statute as written.

Source: Chubbuck City Code Ch. 18.10, verified 2026-07-05.

Coeur d'Alene · by right, but 800 sq ft cap conflicts with state law ⚠ conflict
  • Code: CDA Municipal Code §§ 17.06.650–.670 (Ord. 3639, 2019, amended 2021). ADUs allowed since 2007.
  • One ADU per single-family lot; attached, detached, or basement; in all residential and commercial zones. Administrative permit with a recorded notice that runs with the land.
  • Size: max 800 sq ft and no more than 75% of the primary dwelling.
  • Owner-occupancy only triggers if either unit is used as a short-term rental.
  • ⚠ Vs. state law: the 800 sq ft cap sits below the state's 1,000 sq ft floor, and the one-space parking rule has none of the statute's exceptions. Both conflict with the text.

Source: CDA Municipal Code § 17.06.650 (current through Ord. 3774), verified 2026-07-04.

Eagle · sliding size cap tied to lot size ⚠ conflict? open
  • Code: Eagle City Code § 8-3-5(U).
  • The most size-restrictive standard in the survey: a sliding cap from 500 sq ft on the smallest qualifying lots up to 1,200 sq ft on large lots.
  • ? A confusingly worded 17,000 sq ft lot-size threshold may function as a near-ban on smaller lots; the text reads as internally inconsistent and I flag it as unresolved.
  • Parking follows the city's apartment/multi-family standard rather than a flat one-space rule.
  • ⚠ Vs. state law: the 500 sq ft end of the sliding cap is far below the state floor.

Source: Eagle City Code § 8-3-5 (current through Ord. 970), verified 2026-07-04.

Garden City · by right, up to 1,200 sq ft ✓ clear
  • Code: Garden City Development Code § 8-4B-4 (current through Ord. 1056-25, June 2025).
  • Size: 800 sq ft or 50% of the principal dwelling, whichever is greater, up to a 1,200 sq ft cap. One of the more permissive size rules surveyed.
  • One ADU per property; no ADU-specific setbacks beyond the underlying zone; nothing in front of the principal structure.
  • No parking or owner-occupancy requirement found in the section.

Source: Garden City Development Code § 8-4B-4, verified 2026-07-04.

Hayden · by right, hard 1,000 sq ft cap ? open
  • Code: Hayden City Code § 11-2-5(A).
  • Size: 1,000 sq ft flat cap. An attached unit over 1,000 sq ft reclassifies the whole structure as a duplex.
  • One ADU per lot; no new or separate driveway approach (must share the existing one); one added parking space.
  • ADU setbacks (30 ft front, 10 ft side/rear) are larger than the city's typical accessory-structure setbacks.
  • ? Vs. state law: not yet assessed here. Whether the flat 1,000 sq ft cap satisfies the state's 1,000-or-75% floor depends on a statute reading that's still an open question.

Source: Hayden City Code § 11-2-5, verified 2026-07-04.

Idaho Falls · by right in all residential zones, 750 sq ft cap ⚠ conflict
  • Code: Idaho Falls Municipal Code § 11-2-6(N), "Dwelling, Accessory Unit."
  • Permitted use in all 8 residential zones plus several commercial and industrial zones. Broader zone coverage than most cities surveyed.
  • ⚠ Vs. state law: 750 sq ft size cap and an owner-occupancy requirement enforced by recorded deed restriction. Both conflict with the statute.

Source: Title 11 Ch. 2, city PDF (through Ord. 3629), verified 2026-07-04.

Jerome · special use permit in every zone ⚠ conflict? open
  • Code: JMC § 17.18.020(I), from the ordinance presented at the July 23, 2025 Planning & Zoning hearing. Final council adoption not independently confirmed; treat as very likely current.
  • Size: flat 850 sq ft cap, no percentage alternative.
  • The zoning table marks ADUs "S" (Special Use) in every zone that allows them: a full discretionary process with public hearing and a commission vote.
  • ⚠ Vs. state law: the 850 sq ft cap and the discretionary Special Use process both conflict with the statute.

Source: July 2025 P&Z packet, verified 2026-07-04.

Kuna · special use permit required · survey partial ⚠ conflict◑ partial
  • Confirmed so far: a Special Use Permit is required in the 5 zones that allow ADUs; a 75% size rule, 2-bedroom max, and 1 parking space are confirmed via a code mirror and search.
  • ? Whether Kuna requires owner-occupancy is still unconfirmed. Kuna's code host (Municode) can't be read by my tools, so this entry is partial.
  • ⚠ Vs. state law: the Special Use Permit path conflicts with the matter-of-right requirement.

Sources: Ordinance 2023-14 (PDF), verified 2026-07-04; standards via a zoneomics.com mirror, 2026-07-05.

Lewiston · patchwork rules, 900 sq ft ceiling ⚠ conflict
  • Code: Lewiston Code of Ordinances Ch. 37; the citywide "Accessory Apartments" standard is § 37-131.2, with separate zone-specific versions for R-1 and the two Normal Hill zones. The provision is duplicated or varied across at least 4 code locations.
  • § 37-131.2: 600 sq ft or 30% of the primary, whichever is greater, capped at 900 sq ft; max 2 bedrooms; applies to residential lots of at least 5,000 sq ft with one lawful single-family dwelling.
  • ⚠ Vs. state law: the 900 sq ft ceiling sits below the state floor.

Source: Lewiston's Municode-hosted code read via a zoneomics.com mirror, verified 2026-07-05.

Meridian · owner-occupancy required ⚠ conflict
  • Code: Meridian UDC § 11-4-3-12, "Dwelling, accessory," as amended by Ordinance 25-2102 (November 2025).
  • One ADU per property, subordinate to a single-family dwelling; city permit required; utility-connection and separate-address requirements added in the 2025 rewrite.
  • ⚠ Vs. state law: the owner-occupancy requirement (owner or occupant of one unit must reside on site 6+ months a year) survived the November 2025 rewrite intact and squarely conflicts with the statute.

Source: Meridian Ordinance No. 25-2102 (the ordinance PDF from the October 2025 council packet; no stable public link captured), verified 2026-07-04.

Middleton · by right, but 14,000 sq ft minimum lot ⚠ conflict? open
  • Code: Middleton City Code § 5-4-1, Table 1, "second dwelling unit" (table last amended by Ord. 715, March 2026). Population is close to the 10,000 threshold; coverage worth re-verifying.
  • Listed as an Allowed use (no CUP) in R-1, R-2, R-3, and M-U.
  • ⚠ Vs. state law: a footnote attaches a 14,000 sq ft minimum lot size and a detached-only requirement, which could work as a de facto ban on typical in-town lots.

Source: Middleton City Code § 5-4-1, verified 2026-07-04.

Moscow · 600 sq ft cap, owner-occupancy by deed restriction ⚠ conflict
  • Code: Moscow City Code Title 4, § 3-5(A). Permitted accessory use in all seven residential zones.
  • Size: 600 sq ft or 40% of the principal dwelling, whichever is less. Occupancy capped at 2 persons.
  • Owner-occupancy is mandatory and enforced through a recorded deed restriction. One flat additional parking space, no exceptions.
  • ⚠ Vs. state law: the size cap, the owner-occupancy rule, and the parking rule all conflict with the statute.

Source: Moscow City Code Title 4 Ch. 3 (PDF), verified 2026-07-04.

Mountain Home · no ADU standard found in the code – none? open
  • No standalone ADU standard was found: no entry in the Land Use Chart (§ 9-7-4), no definition (§ 9-3-2), no parking line item (§ 9-12-4). The only mention is one passing phrase, "accessory dwelling unit where allowed," in the generic accessory-building section (§ 9-9-2).
  • ? That's a genuine gap, not a restrictive rule. How the city actually handles an ADU application is an open question I've flagged for a phone follow-up.

Source: Mountain Home City Code (current through Ord. 1814), verified 2026-07-04.

Nampa · no true ADU ordinance, in-law quarters only ⚠ conflict
  • Nampa's code still uses the older concepts of attached "father-/mother-in-law quarters" and detached "guesthouses," both deliberately designed as dependent space: no separate address, no separate mailbox, and (for the attached type) no 220-volt outlet and no split electrical panel.
  • Both types are listed as permitted uses across most residential zones (Title 10, § 10-3-2).
  • ⚠ Vs. state law: this framework doesn't produce the independent dwelling unit the statute defines. Likely the largest compliance gap of any city surveyed.

Source: Nampa § 10-3-2 land use schedule (PDF), verified 2026-07-04.

Pocatello · 750 sq ft, one bedroom, owner-occupancy ⚠ conflict
  • Code: Pocatello City Code § 17.06.100. One ADU per single-family lot, allowed in all residential, commercial, and mixed-use zones.
  • Size: 750 sq ft cap and a one-bedroom max. Owner-occupancy enforced by recorded deed restriction.
  • Parking is conditional and can reach two spaces if on-street parking isn't available.
  • ⚠ Vs. state law: the size cap, bedroom limit mechanism, owner-occupancy rule, and parking rule all sit on the restrictive side of the statute.

Source: Pocatello City Code § 17.06.100, verified 2026-07-04.

Post Falls · owner-occupancy, parking, and size all conflict with state law ⚠ conflict
  • Code: PFMC 18.24.030(A)(3). One ADU per single-family lot; basement, attached, or detached.
  • As written the code requires owner-occupancy, one additional paved off-street parking space, and caps size at the smaller of 50% of the primary home or 1,000 sq ft.
  • ⚠ Vs. state law: all three of those provisions conflict with the statute. Full analysis: the state law vs. the Post Falls code.
  • This is my city. The Post Falls build tracker follows every question and cost of a real ADU project here.

Sources: PFMC 18.24.030 and the city ADU page, verified 2026-07-03.

Rathdrum · CUP only, no new detached units · state-law coverage disputed ⚠ conflict? open
  • ? Whether the state law covers Rathdrum at all is disputed: the 2020 census counted 9,289 (under the line) while recent estimates run near 10,874 (over it). Which population figure controls is an open question.
  • Code: Rathdrum City Code 15-6-2-J. ADUs require a Conditional Use Permit, and the code does not allow a wholly new detached ADU. Units must come from converting or expanding existing space.
  • Size: between 400 and 800 sq ft (the lesser of 75% of the primary's footprint or 800). One bedroom max. Owner-occupancy required.
  • Timing gates: no ADU within 2 years of the primary residence's certificate of occupancy, and none in a subdivision where more than 15% of lots remain undeveloped.

Source: Rathdrum City Code 15-6-2-J, located via the Housing Solutions Partnership white paper, verified 2026-07-05.

Rexburg · conditional use permit in every zone · survey partial ⚠ conflict◑ partial
  • Confirmed so far: a Conditional Use Permit is required in every zone that allows ADUs. No by-right path.
  • ? The numeric standards (size, parking, owner-occupancy) haven't been located yet; Rexburg's code host can't be read by my tools, so this entry is partial.
  • ⚠ Vs. state law: a CUP-only path conflicts with the matter-of-right requirement.

Source: CUP requirement confirmed via a zoneomics.com mirror of the city code, as of 2026-07-05.

Star · owner-occupancy, 10,000 sq ft minimum lot ⚠ conflict
  • Code: Star City Code § 8-5-30, "Secondary Dwelling Unit."
  • Detached units capped at 50% of the primary dwelling's footprint. Minimum 10,000 sq ft property. One added parking space. Occupancy capped at 2 persons.
  • Owner-occupancy required: the owner must reside on the property more than 6 months per year.
  • ⚠ Vs. state law: the footprint cap, lot minimum, and owner-occupancy rule all conflict with the statute.

Source: Star's Municode-hosted § 8-5-30 read via a zoneomics.com mirror, verified 2026-07-05.

Twin Falls · already matches the state law ✓ clear
  • Code: Twin Falls UDC § 10-6-10(B), part of the brand-new Title 10 adopted December 8, 2025, effective January 1, 2026. It predates the state law entirely.
  • Size floor, parking exception logic, height tied to the primary dwelling, and administrative (not discretionary) approval are all already in place. Nearly point for point the statute's requirements.
  • The strongest already-compliant city in the whole 25-city survey, ahead of Boise and Caldwell.

Source: Twin Falls Title 10 (§ 10-6-10(B), pp. 102–103 of the city PDF), verified 2026-07-04, re-confirmed 2026-07-05.

In a smaller city or town · state law does not apply

Towns under 10,000 people are not covered by § 67-6541. The town's own municipal code is the whole story, and it may restrict or prohibit ADUs entirely. I've surveyed the small towns of Kootenai County so far, using the Housing Solutions Partnership's ADU guide white paper addendum (rev. 2025-05-02). That's a volunteer-committee survey, not primary code text, so treat these as starting points (all as of 2026-07-05):

  • Athol: not allowed, and the reason isn't zoning. The town has no municipal sewer, and under its sewage management agreement with Panhandle Health District most lots carry a single wastewater allotment. An ADU needs a second one.
  • Dalton Gardens: city code prohibits ADUs outright.
  • Fernan Lake Village: has an ADU code, including rules on rental use; its terms weren't detailed in the source.
  • Harrison: allowed, but each one takes City Council approval. Not allowed in every part of town.
  • Spirit Lake: actively drafting an ordinance to allow ADUs as part of a comprehensive plan update.
  • Hauser Lake: considering ADUs, no decision yet.

Your town isn't listed, or is listed without an answer? There is no state-law backstop under 10,000 people, so your town's own code is the whole story. Tell me your town and I'll research it and add it to this page.

On unincorporated county land · county code governs · 23 of 44 counties surveyed

The state ADU law does not touch county zoning at all, regardless of the county's size. Your rights come entirely from the county's own land use code, and some rural counties barely have one. One nuance: land inside a city's Area of City Impact is still unincorporated and still under county zoning, but the county/city agreement for that area can impose city-like standards (Idaho Code § 67-6526). I've surveyed 23 of Idaho's 44 counties so far.

Ada County · secondary dwellings up to 1,200 sq ft ✓ clear? open
  • Code: Ada County Code Title 8, § 8-4-23, "Dwelling, Secondary Attached or Detached."
  • Allowed on any property with a valid principal dwelling; one additional dwelling per property. Size: 350–1,200 sq ft or 60% of the principal dwelling's above-grade living space, whichever is less. 3-bedroom cap. Minimum 6,000 sq ft property.
  • Occupancy on the property is required (principal or secondary, not necessarily the owner), with a recorded deed restriction that converts the unit to non-habitable space if that lapses.
  • The source document is marked as a consolidated draft; I've flagged confirming it matches the adopted version.

Source: Ada County Zoning Ordinance Title 8 (county document), verified 2026-07-04. The county's public code library hosts the current code (link confirmed 2026-07-13).

Bingham County · 2-acre minimum, caretaker framing ✓ clear
  • Code: Bingham County Code Title 10, § 10-7-2, "Accessory Structure With Living Space And Caretaker's Residence."
  • One per buildable residential parcel with a building permit, on a minimum 2-acre parcel (waivable by CUP). Capped at 50% of the gross floor area (also waivable by CUP). Cannot be sold separately.
  • Separate provisions exist for farm-help dwellings (5+ acre operations, employees only) and temporary medical-hardship dwellings.

Source: Bingham County Code Title 10 (county document; no stable public link captured), verified 2026-07-05.

Blaine County · rights scale with acreage ✓ clear
  • Code: Blaine County Code Title 9, § 9-3-11 (current through Ord. 2025-08). The most detailed county ADU ordinance in this survey.
  • Under 1 acre: CUP required, and only on a regulated community water system with health-district approval. 1–2 acres: CUP. 2–5 acres: one ADU by right. 5+ acres: one by right, a second possible via CUP.
  • Size: 1,200 sq ft cap, plus up to 1,200 additional sq ft of garage that doesn't count against it.
  • No owner-occupancy requirement and no rental restriction found.

Source: Blaine County Code § 9-3-11, verified 2026-07-05.

Bonner County · by right nearly everywhere, 900 sq ft ✓ clear
  • Code: Bonner County Revised Code Title 12, § 12-490.
  • One ADU per lot, permitted (not conditional) across nearly every zoning district. 900 sq ft cap, 2 bedrooms, 1 added parking space, separate outside entrance, same setbacks as a regular dwelling.
  • On lots under 20,000 sq ft the unit must be attached to the primary structure or a garage.
  • No owner-occupancy requirement and no rental restriction found.

Source: Bonner County Planning Department ADU standards handout, rev. March 2024, verified 2026-07-05. The county's public code library hosts BCRC Title 12 (link confirmed 2026-07-13).

Bonneville County · no ADU provision found – none? open
  • No ADU, secondary-dwelling, or equivalent provision was found in the zones checked (A-1, RA-1, RA-2), which allow only one-family dwellings plus accessory buildings. The one exception is a narrow farm-labor dwelling allowance in A-1.
  • ? Not yet confirmed across every remaining zone; treat as a strong negative finding pending that check.

Source: Bonneville County Code of Ordinances Ch. 156 (no stable public link captured), verified 2026-07-05.

Butte County · no general ADU path – none
  • No general ADU or secondary-dwelling concept for ordinary residential lots. The only related provisions are conditional uses for farm-related additional dwellings (Agricultural zone) and security residences (Industrial zone).

Source: Butte County Zoning Ordinance No. 3-99, read in full (no stable public link captured), verified 2026-07-05.

Camas County · no size limit, 2.5-acre minimum ✓ clear
  • Code: Camas County Code § 2-2-2 and Ch. 8 § C(2), "Additional Dwelling Unit" (detached only).
  • Allowed in the Agricultural (A-2.5) and Agricultural Transition districts on lots of 2.5 acres or larger, with South Central Public Health District approval.
  • The ordinance states plainly that there is no limit on the size of the unit. The most permissive size rule found in the survey. No owner-occupancy, rental, or parking rule found.

Source: Camas County Zoning Ordinance (county document; no stable public link captured), verified 2026-07-05.

Canyon County · two conditions only ✓ clear
  • Code: Canyon County Code § 07-14-25, "Secondary Residence" (as amended by Ord. 16-001, 2016).
  • The current standard has exactly two conditions: the property owner must live on site, and the secondary residence must have its own address for emergency purposes. A 2016 amendment genuinely repealed the older size cap and distance limit (confirmed two independent ways).

Source: Canyon County Code § 07-14-25, American Legal Publishing, verified 2026-07-04, re-confirmed 2026-07-05; library link confirmed 2026-07-13.

Clearwater County · two dwellings per parcel, no ADU category ✓ clear
  • No distinct ADU concept. Instead a county-wide rule (§ 501.3(2)) allows up to two dwellings on any single-deed parcel.
  • The second dwelling is treated as an ordinary dwelling under the same zone standards as the first: no reduced size cap, no reduced parking, no owner-occupancy condition.

Source: Clearwater County Zoning Ordinance Art. 5 and Land Use Ordinance No. 51 (county documents; no stable public link captured), verified 2026-07-05.

Custer County · by right in ag zones, conditional in residential ✓ clear
  • Code: Custer County Zoning Ordinance No. 2007-09 (amended through 2019), "Accessory Dwelling."
  • Permitted by right in the Agricultural, Industrial, and Commercial zones but a discretionary Conditional Use in the Transitional-Agricultural and Residential zones. The reverse of the usual pattern.
  • No ADU-specific size cap, parking, or siting standard anywhere in the ordinance; only the general dimensional standards apply.

Source: Custer County Zoning Ordinance 2007-09 (county PDF; no stable public link captured), verified 2026-07-05.

Elmore County · 900 sq ft, rentable, no owner-occupancy ✓ clear
  • Code: Elmore County Zoning and Development Ordinance Title 7 (ADU defined at § 7-2-96).
  • One ADU per legal lot, approved administratively, in any zone except industrial/commercial. 900 sq ft and 2-bedroom cap. May be a conversion, addition, or new detached structure.
  • The ordinance says explicitly the ADU may be rented out. No owner-occupancy requirement and no ADU-specific lot-size minimum. One added parking space; neighbors within 300 feet get notice.
  • Application fee: $150, confirmed unchanged from 2021 through the current 2026 fee schedule.

Source: Elmore County Zoning Title 7 (county PDFs; no stable public link captured), verified 2026-07-04.

Fremont County · family or employees only, no open-market rental ✓ clear
  • Code: Fremont County Development Code (2011), Appendix U § 1.43, "Dwelling, Secondary Attached Or Detached," plus the density mechanism at § 5.55. Structurally near-identical to Ada County's ordinance.
  • Class I administrative permit in at least four rural zones. Size: lesser of 900 sq ft or 60% of the primary's footprint. 6,000 sq ft minimum property if attached.
  • One of the two dwellings must be owner-occupied, and the secondary dwelling may only house the owner, immediate family, or the owner's employee. No open-market rental. Annual compliance statement required.

Source: Fremont County Development Code 2011 (county document; no stable public link captured), verified 2026-07-05.

Gem County · 1,800 sq ft cap, $40 application ✓ clear
  • Code: Gem County Code Title 11, § 11-6-5(L), "Dwelling, Accessory."
  • One accessory dwelling per property, attached or detached, in any zone that allows residential use. 1,800 sq ft gross floor area cap.
  • No owner-occupancy requirement and no rental restriction. A recorded deed restriction (no separate sale, size cap) is required before the building permit.
  • Administrative process, $40 application fee (separate from the building permit). Wastewater must be approved by Southwest District Health.

Source: Gem County application packet reproducing the ordinance text, verified 2026-07-04. The county's public code library hosts Title 11 (link confirmed 2026-07-13).

Idaho County · effectively no zoning ✓ clear
  • Idaho County's entire Planning and Zoning Ordinance (No. 67) classifies all unincorporated land as "multi-use with no restrictions" beyond other applicable ordinances. No zone map, no use table, no size or setback standards.
  • So there is no ADU provision because there is effectively no zoning restriction to need an exception from. Building, septic, and other non-zoning rules still apply.

Source: Idaho County Ordinance No. 67 (2020-02-18; county document), verified 2026-07-05. The county's public code library is online (link confirmed 2026-07-13).

Kootenai County · one ALU per parcel, aquifer rules matter ✓ clear
  • Code: Land Use and Development Code Title 8. The county's term is "Accessory Living Unit" (ALU); the section is unchanged since Ord. 514 (2017).
  • One ALU per parcel in every residential zone. Size: 1,000 sq ft or 50% of the primary home's habitable space, whichever is greater. No owner-occupancy requirement.
  • The Agricultural and Rural zones require at least 2 acres for an ALU; the denser residential zones have no acreage minimum.
  • Over the Rathdrum Prairie Aquifer, Panhandle Health District's one-dwelling-per-5-acres density rule effectively requires a 10-acre parcel for an ALU. The health district binds harder than the zoning code there.
  • Permitting is administrative, $85, reviewed with the building permit.

Source: Kootenai County Land Use and Development Code Title 8, American Legal Publishing (current through Ord. 620), verified 2026-07-04; library link confirmed 2026-07-13.

Lemhi County · no ADU provision found – none
  • No accessory- or secondary-dwelling provision found in the county's Development Code (2022 edition), after keyword searches and a full scan of every division heading. A real, fairly detailed code that simply doesn't define this use.

Source: Lemhi County Development Code 2022 (county document; no stable public link captured), verified 2026-07-05.

Lewis County · no ADU category; duplexes by right – none
  • No ADU concept. The whole county is one "Multiple Use Zone District," and its use table lists one- and two-family dwellings as allowed by right. A duplex built from the outset works as a partial ADU-equivalent.
  • A detailed Temporary Hardship Residence provision (§ 5-8) allows a temporary second dwelling for a dependent family member: 1-acre minimum lot, physician's statement, renewal every 2 years, removed when the hardship ends.

Source: Lewis County Zoning and Development Ordinance No. 97-2, read in full (county PDF; no stable public link captured), verified 2026-07-05.

Nez Perce County · by right in five zones ✓ clear
  • Code: Land Development Code Title 110; the county's term is "Accessory apartment" (§ 110.16.060).
  • Permitted by right (no CUP) in five zones: Agricultural, Agricultural-Residential, Forest, Rural Residential, and Light Commercial/Industrial. One per lot.
  • Size: the greater of 700 sq ft or 30% of the dwelling's gross floor area. Max 2 bedrooms. Septic approval and a certificate of occupancy required.
  • No owner-occupancy requirement, no rental restriction, no ADU-specific parking rule. One of the most accessible counties surveyed.

Source: Nez Perce County LDC Title 110, via a zoneomics.com mirror cross-referenced with Municode's listing, verified 2026-07-05.

Owyhee County · survey partial, Marsing-area overlay only ◑ partial? open
  • Confirmed so far: in the Marsing Area of City Impact overlay's Residential district (§ 8-2C-4-2), attached family-member quarters with a common wall are allowed by right. That's family-only and attached-only, not a general rentable ADU, and it applies only in the Marsing area.
  • The countywide Title 9 zoning ordinance's own ADU provision (if one exists) hasn't been located yet. This entry is partial.

Source: Owyhee County Code, verified 2026-07-05.

Payette County · agricultural zone only ✓ clear
  • Code: Title 8 (Zoning); ADU last amended by Ord. 2021-04.
  • One ADU per parcel, but only as a listed use in Zone A (Agricultural). Not a listed use in the Residential, Recreational, or commercial/industrial zones.
  • In Zone A: minimum 2-acre tract with an existing single-family dwelling, 1,500 sq ft cap, built within 200 feet of the primary, shared driveway and street address.
  • No owner-occupancy requirement in the ADU text. Permitting runs through the standard building-permit process; septic review by Southwest District Health runs $90–$235.

Source: Payette County Title 8, American Legal Publishing (2025 S-7 supplement), verified 2026-07-04; library link confirmed 2026-07-13. Septic fees: SWDH.

Teton County · 1-acre minimum, cap lifts at 5 acres ✓ clear? open
  • Code: Teton County Code Title 8, §§ 8-4-2 and 8-6-2(B)(15) (code text dated 2013, the oldest in this survey).
  • One accessory unit per parcel; 1-acre minimum parcel just to have one. Size: the greater of 50% of the primary or 900 sq ft, up to 1,500 sq ft. Parcels of 5+ acres are exempt from the size cap entirely.
  • Permitted with conditions in the Agricultural zones and R-1; conditional use in R-2. Two parking spaces required in the Agricultural zones.
  • No owner-occupancy requirement found.

Source: Teton County land development code (county document; no stable public link captured), verified 2026-07-05.

Twin Falls County · no rear-yard second dwelling; guesthouses allowed ? open
  • Code: Title 8, §§ 8-2-2 and 8-12-5 (current through Ord. 292, December 2025). Not the same code as the City of Twin Falls.
  • § 8-12-5(B), "Rear Dwellings," bars building or converting a second dwelling behind the principal building on the same lot. That's a siting restriction, not an outright ADU ban (an earlier version of my survey got this wrong; corrected 2026-07-05).
  • A separate "Guesthouse" category allows up to 3 bedrooms but cannot be rented.

Source: Twin Falls County Code Title 8, verified 2026-07-05.

Valley County · permitted countywide, owner-occupancy required ✓ clear
  • Code: Valley County Code § 9-4-7 (current through Ord. 2025-07, December 2025, the most recently amended county ADU ordinance surveyed).
  • One ADU as a permitted use in all zones countywide. Size: lesser of 1,000 sq ft or 50% of the primary structure. Same setbacks as the primary; one dedicated parking space.
  • Owner-occupancy required for one of the two residences. Long-term rental (30+ days) explicitly allowed; short-term rental requires a CUP.
  • Administrative process: $75 fee, 300-foot neighbor notice, 10-day objection window. Septic/water approval by Central District Health.

Source: Valley County Code § 9-4-7, American Legal Publishing, plus the county ADU application, verified 2026-07-04; library link confirmed 2026-07-13.

Not yet surveyed (21 counties): Adams, Bannock, Bear Lake, Benewah, Boise, Boundary, Caribou, Cassia, Clark, Franklin, Gooding, Jefferson, Jerome, Latah, Lincoln, Madison, Minidoka, Oneida, Power, Shoshone, and Washington. These get added as the survey continues. If your county is on this list, start with its planning and zoning office, or tell me your county and I'll move it up the list.

Layer 3: the rules that apply everywhere

No matter which branch above you're in, these still bind:

This page is research, not legal advice. Every fact carries the date I verified it, and city codes are actively changing until the February 1, 2027 compliance deadline. If a number here decides your project, click through to the source and re-verify it, or call the planning department. Corrections welcome.

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