Idaho ADU Laws: Your Complete Guide

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.

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.

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:

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.

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.