Idaho ADU Laws: Your Complete Guide
Idaho law protects your right to build an ADU. Here is exactly what the state law says, what Twin Falls code requires, and what your HOA can and cannot do.
Quick Answer
Idaho ADU law runs on two separate tracks. Senate Bill 1354 (signed March 31, 2026, effective July 1, 2026, Idaho Code §67-6541) is the law that actually requires cities to allow ADUs — but only cities over 10,000 population. In the Magic Valley that's Twin Falls, Jerome, and Burley, which must allow at least one ADU per single-family lot by-right and update their codes by February 1, 2027. House Bill 166 (2023, Idaho Code §55-3212) is a separate HOA-only law — it stops a homeowners association from banning an internal ADU on an owner-occupied homestead, but it does NOT bind cities or counties. Twin Falls already permits ADUs under Title 10, Section 10-6-10-B: 45% size rule, design consistency, $250 Zoning Use Permit.
The Short Version
- ✓Idaho Senate Bill 1354 (signed March 31, 2026, effective July 1, 2026) requires Idaho cities over 10,000 population to allow at least one ADU per single-family lot by-right. In the Magic Valley that's Twin Falls, Jerome, and Burley.
- →Smaller cities (Rupert, Filer, Buhl, Kimberly, Gooding, Wendell, Hagerman) and unincorporated county land are not bound by SB 1354 — there, ADU rights depend on the local zoning code.
- ✓Idaho HB 166 (2023) is an HOA-only law: it stops a homeowners association from banning an internal ADU on an owner-occupied homestead. It does NOT bind cities or counties.
- ✓Twin Falls already allows ADUs under Title 10, Section 10-6-10-B — and as an over-10,000 city must align that code with SB 1354 by February 1, 2027.
- →Local rules on size, setbacks, design, and permits still apply within the framework of state law.
Idaho Senate Bill 1354 — The 2026 Statewide ADU Law
Senate Bill 1354 is the most significant Idaho ADU law in over a decade. Governor Brad Little signed it on March 31, 2026; it takes effect July 1, 2026; and it is codified at Idaho Code §67-6541. The bill passed the Idaho Senate 25 to 10 and the House 47 to 23 before reaching the governor's desk.
What SB 1354 Requires
SB 1354 applies to Idaho cities with a population greater than 10,000. Twin Falls (~52,000 population) qualifies, as do Boise, Nampa, Meridian, Idaho Falls, Pocatello, Caldwell, Coeur d'Alene, and Post Falls.
Affected cities must:
- Allow at least one ADU per single-family residential lot by right (not as a special-use exception requiring case-by-case approval)
- Not use zoning rules to effectively block ADUs in residential zones
- Honor specific limits on what restrictions cities can impose around parking, impact fees, ADU size, owner-occupancy requirements, and building height — relative to what applies to the primary dwelling on the same lot
- Amend their comprehensive plans and land-use regulations by February 1, 2027 to bring local codes into compliance with the state framework
What This Means for Twin Falls Homeowners
Twin Falls already permits ADUs under Title 10, Section 10-6-10-B of the Uniform Development Code — so unlike some Idaho cities, Twin Falls isn't starting from scratch. The current code allows ADUs on any legal single-family lot, applies the 45% size rule, requires design consistency with the primary dwelling, and charges a $250 Zoning Use Permit fee.
Between now and February 1, 2027, the City of Twin Falls will be reviewing Title 10-6-10-B against the SB 1354 framework. Specific rules around parking (currently one extra space required), owner-occupancy, and impact fees may shift to align with the state minimums. Homeowners with active or planned ADU projects should expect the regulatory environment to become more permissive, not less.
Why This Is Time-Sensitive
SB 1354 was passed in response to Idaho's ongoing housing supply crunch. The legislature recognized that ADUs are one of the fastest, lowest-disruption ways to add housing units in already-developed neighborhoods, and that some Idaho cities were dragging their feet on ADU permitting. The bill takes the right to build out of local political deliberation and into state law.
For homeowners considering an ADU, this means: the rules are stabilizing in your favor. Cities that have been restrictive will be required to loosen up. The window between July 2026 and February 2027 is essentially a transition period where Twin Falls' existing code remains in effect while the city works on updates. Starting your project now means you benefit from both the current code and the more permissive 2027 framework when your project closes.
Source: Idaho State Legislature — SB 1354 bill page · Official text: S1354.pdf · Last updated 2026-05-29.
Idaho House Bill 166 (2023): An HOA Law, Not a Zoning Law
There is a widespread misconception — repeated on a lot of ADU websites — that Idaho HB 166 forces cities to allow ADUs. It does not. HB 166 (codified at Idaho Code §55-3212 and §55-618) is a homeowners-association law, not a zoning law. The version of the bill that would have stopped cities and counties from banning ADUs was removed in the Idaho Senate before the bill passed.
What HB 166 actually does: it prohibits a homeowners association (or any private deed restriction) from banning an “internal” accessory dwelling unit — a self-contained unit with its own cooking, sleeping, and bathroom facilities built inside an owner-occupied home or its attached/detached garage — on a detached, owner-occupied homestead. It protects at most one such internal unit per homestead and took effect July 1, 2023.
What HB 166 does NOT do: it does not bind cities or counties, does not override local zoning, and does not grant any statewide right to build an ADU. So if you are asking “can I build an ADU in my city,” HB 166 is not the answer — your city's zoning code (and, for larger cities, SB 1354 below) is.
What HB 166 Does:
- ✓Stops an HOA from banning an internal ADU on a detached, owner-occupied homestead
- ✓Makes post-July-1-2023 HOA covenants that prohibit an internal ADU void and unenforceable (with attorney-fee shifting)
- ✓Protects at most one internal ADU per homestead
What HB 166 Does NOT Do:
- →Does NOT bind cities or counties or override local zoning
- →Does NOT cover detached ADUs, RVs, or tiny homes on wheels — internal units only
- →Does NOT reach covenants entered before July 1, 2023 (not retroactive)
Idaho Senate Bill 1354 (2026): What Actually Requires Cities to Allow ADUs
SB 1354 (signed March 31, 2026, effective July 1, 2026, codified at Idaho Code §67-6541) is the law that genuinely requires Idaho cities to permit ADUs — but it only binds cities with a population over 10,000. In the Magic Valley and Wood River Valley, that means Twin Falls (≈54,900), Jerome (≈13,100), and Burley (≈11,700). Every other city we serve — Rupert, Filer, Buhl, Kimberly, Gooding, Wendell, Hagerman, plus Bellevue, Hailey, and Ketchum — is under the threshold and is NOT bound by SB 1354.
For bound cities (Twin Falls, Jerome, Burley), SB 1354 requires:
- Allowing at least one ADU per single-family lot — either one internal ADU or one detached ADU — as a by-right use approved administratively, without discretionary review
- No ADU bans in any residential zone
- Code amendments completed by February 1, 2027
SB 1354 also caps what bound cities can require. They may NOT:
- Require off-street parking for the ADU (with narrow exceptions)
- Charge impact or utility-connection fees greater than for other single-family dwellings
- Limit ADU size below 1,000 sq ft or 75% of the primary dwelling (whichever is larger)
- Require owner-occupancy
- Limit ADU height below the primary dwelling, or impose stricter setback/lot-size/coverage rules than for single-family homes in the same district
Cities may always adopt less restrictive rules, and SB 1354 does not override building, fire, floodplain, or water/sewer safety codes. For Twin Falls homeowners this is unambiguously good news: between the existing Title 10-6-10-B allowance and the SB 1354 mandate, your right to build an ADU is more protected than ever — and several current Twin Falls restrictions (parking, owner-occupancy) may loosen as the city aligns its code by February 2027.
Sources: Idaho Legislature — SB 1354 (Idaho Code §67-6541) · HB 166 (Idaho Code §55-3212). Last verified 2026-06-01.
HOAs and ADUs: What HB 166 Protects
If you live in an HOA community, HB 166 is the law that matters for you. As of July 1, 2023, an HOA cannot create or enforce a covenant that bans an internal ADU (a self-contained unit inside your owner-occupied home or its garage). Pre-July-1-2023 covenants are not reached by the law, and the protection is for owner-occupied homesteads only.
If your HOA pushes back on an internal ADU on your owner-occupied property, those covenant provisions may be void and unenforceable — and the statute provides for attorney-fee recovery. That said, HOAs may still enforce reasonable design and dimensional rules, and HB 166 does NOT protect detached ADUs from HOA covenants. If your HOA disputes your ADU, consult an Idaho real estate attorney.
What the HOA Protection Covers:
- ✓HOAs cannot ban an INTERNAL ADU on an owner-occupied, detached homestead
- ✓Post-July-1-2023 covenants prohibiting an internal ADU are void/unenforceable (with fee-shifting)
- ✓Applies to owner-occupied, detached single-family homesteads
What the HOA Protection Does NOT Cover:
- →Detached ADUs, RVs, or tiny homes on wheels (internal units only)
- →Covenants entered before July 1, 2023 (not retroactive)
- →Reasonable HOA rules on size, height, setbacks, parking, and design review
Twin Falls ADU Regulations: Title 10, Section 10-6-10-B
The City of Twin Falls regulates ADUs through Title 10 of the Uniform Development Code. Here is a summary of the key requirements:
Who Can Build
ADUs are permitted on any legal lot containing a legal single household dwelling unit. Only one ADU is allowed per lot or parcel.
45% Size Rule
The ADU cannot exceed 45% of the living space of the principal residence. Example: 2,000 sq ft home = max 900 sq ft ADU. The 45% is calculated on living space, not total footprint (garage/unfinished areas excluded).
Setbacks — Detached Under 15 ft
20 ft front, 3 ft side, 3 ft rear. Applies across all zoning districts (R-2, TN-1, TN-2, TN-3, RR). Structures with vehicle access must be 20 ft from public streets, 10 ft on alleys.
Setbacks — Detached Over 15 ft
TN-1: 20' front, 7' side, 20' rear (max 35'). TN-2 and TN-3: 20' front, 5' side, 15' rear (max 35'). All other districts: 20' front, 5' side, 15' rear (max 35').
Design Match Required
The ADU must be consistent with the existing roof pitch, siding, and windows of the principal dwelling. This applies to attached, detached, and converted structures alike.
Shared Utilities (No Separate Meters)
Per Title 10-6-10-B(vii): the ADU must receive water, sewer, and sanitation from the primary dwelling. Additional service lines and/or meters shall not be installed.
Parking
One additional off-street parking space is required for exclusive use of the ADU.
Zoning Use Permit
A $250 Zoning Use Permit is required from the Twin Falls Planning and Zoning Department (203 Main Ave E, 208-735-7267, tfplanning@tfid.org). A 7-step review process includes a 15-day appeal window.
ADU Laws in Other Magic Valley Cities
Every city in the Magic Valley has its own zoning code and permit process. The SB 1354 mandate only binds the larger cities (Twin Falls, Jerome, Burley); in the smaller cities, whether and how you can build an ADU is governed entirely by the local code. The specific setbacks, size limits, and fees differ by jurisdiction — we know all of them.
Law Is One Thing. Your Specific Lot Is Another.
Idaho law gives you the right to apply for an ADU permit. Whether your specific lot, zoning district, and property configuration actually support a buildable ADU is a different question — and the answer depends on setbacks, size limits, utility capacity, and access. That's exactly what our Feasibility Check is designed to answer.
HB 166 in Practice — What It Means for Real Homeowners
The text of Idaho House Bill 166 is straightforward: no city or county may impose an outright ban on accessory dwelling units. But what does "outright ban" actually mean in practice, and how do you know if your local rules are HB 166-compliant?
What HB 166 prohibits
- A flat ban on ADUs across all residential zones
- A ban that applies only to detached ADUs (forcing all ADUs to be attached)
- A ban that applies only to ADUs intended for rental
- De facto bans created through unreasonable setbacks, lot sizes, or other rules that make ADUs impossible to build on typical residential lots
What HB 166 still allows local governments to do
- Set size limits (Twin Falls' 45% rule is HB 166-compliant)
- Require design consistency with the primary dwelling
- Require permits and inspections
- Set setback and lot-coverage requirements per zoning district
- Restrict short-term rental (Airbnb-style) use, separately from long-term rental
- Charge reasonable permit fees
In short: HB 166 protects your right to build an ADU, not your right to build any particular ADU. Local rules on size, setbacks, and design still apply.
The 2023 HOA Amendment — What Changed
Effective July 1, 2023, an amendment to HB 166 added homeowners associations (HOAs) to the list of entities that cannot ban ADUs. The amendment applies to detached, owner-occupied properties — meaning if you own and live in your home, your HOA cannot enforce a covenant that generally prohibits you from building an ADU.
Important nuances:
- The protection applies to owner-occupied properties. Investment properties or rentals are not covered.
- HOAs may still enforce reasonable architectural standards — e.g., requiring your ADU design match the primary residence (which Twin Falls code already requires anyway).
- HOAs may still enforce design review processes — you may need to submit plans for HOA approval in addition to city permits.
- The protection is for detached properties — attached townhomes or condos may have different rules.
We've worked with multiple Twin Falls and Magic Valley homeowners in HOA communities who had been told for years they couldn't build an ADU. Post-2023, that's no longer accurate — though most HOAs haven't updated their CC&Rs to reflect the change. If you're in an HOA, plan for design review on top of the city permit process.
Twin Falls Historical Context
The City of Twin Falls codified its ADU ordinance under Title 10-6-10-B as part of its Uniform Development Code. The code's stated purposes include providing rental income, companionship, and security for seniors, single parents, and families; increasing the housing stock with affordable housing options; and protecting neighborhood stability and character.
The 45% size rule was set as a compromise between ADU advocates pushing for larger units and traditional zoning concerns about preserving single-family character. In practice, the rule means a typical 1,800-2,200 sq ft Twin Falls home can support an 810-990 sq ft ADU — large enough for a 1-bedroom or compact 2-bedroom unit. The rule scales naturally with home size, so larger homes can support larger ADUs.
The utility-sharing requirement (Section 10-6-10-B(vii)) is somewhat unusual compared to other Idaho jurisdictions, which sometimes allow separate meters. The Twin Falls rule simplifies utility infrastructure but does require the existing primary dwelling's service capacity be sufficient for both units — a Feasibility Check item we always verify before construction.
Questions About Your Specific Property?
Book a free 10-15 minute Readiness Call. We'll review your zoning district, check the 45% size rule, and give you a clear picture of what Idaho law — and Twin Falls code — actually allows on your lot.