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.
The Short Version
- ✓Idaho HB166 prohibits cities and counties from banning ADUs outright.
- ✓As of July 2023, HOAs cannot enforce rules that ban ADUs on owner-occupied properties.
- ✓Twin Falls allows ADUs on any legal single-family lot under Title 10, Section 10-6-10-B.
- →Local regulations still apply — setbacks, size limits, design requirements, and permits are required.
Idaho House Bill 166: The Statewide ADU Law
Idaho House Bill 166 is the foundational law protecting every Idaho homeowner's right to build an ADU. In plain terms, it prohibits any city or county government from imposing an outright ban on accessory dwelling units in residential zones.
Before HB166, some Idaho municipalities refused to process ADU permit applications or had effectively banned them through overly restrictive zoning. HB166 changed that — every Idaho city must now have a process for ADU permits.
Importantly, HB166 does not remove all local control. Cities can still regulate ADUs through setbacks, size limits, design standards, parking requirements, and permitting fees. What they cannot do is ban them entirely.
What HB166 Does:
- ✓Prohibits cities and counties from imposing an outright ban on ADUs
- ✓Requires every Idaho municipality to have a permit process for ADUs
- ✓Applies statewide — Twin Falls, Jerome, Burley, and every other Idaho city
What HB166 Does NOT Do:
- →Does not remove local setback, size, design, or parking requirements
- →Does not guarantee your specific lot is eligible (zoning still matters)
- →Does not override local permit fees or application processes
HOA and ADUs: The 2023 Idaho Law Change
As of July 1, 2023, Idaho law prohibits HOAs from creating or enforcing rules that ban ADUs on detached, owner-occupied properties. This was a significant update that directly addressed a common barrier for homeowners in HOA communities.
If you live in an HOA and your CC&Rs say you cannot build an ADU — those rules may no longer be enforceable if your property is owner-occupied and detached. That said, HOAs may still have some authority over design, aesthetics, and process. If your HOA pushes back, consult an Idaho real estate attorney.
What the HOA Law Covers:
- ✓HOAs cannot create new rules banning ADUs on owner-occupied detached properties
- ✓Existing CC&R provisions that ban ADUs are generally unenforceable on owner-occupied properties
- ✓Applies to owner-occupied, detached (single-family) residential properties
What HOAs May Still Do:
- →Regulate design and aesthetics of the ADU (exterior materials, colors)
- →Require architectural review before construction
- →Set rules for attached or condo-style properties (the law focuses on detached)
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. Idaho HB166 applies statewide, but the specific setbacks, size limits, and requirements 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.
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.