Building an ADU in Idaho: what state law allows

What state law lets you build

SB 1354 passed the Senate 25-10 and the House 47-23 and was signed by the governor on March 31, 2026, as Session Law Chapter 265. It took effect July 1, 2026.

The new Idaho Code 67-6541 says: "No city shall enact or enforce any ordinance, rule, regulation, or policy that bans accessory dwelling units in any residential zoning area within its jurisdiction." An ADU is a residential land use for zoning purposes, subject to applicable zoning rules.

The law defines an ADU as a self-contained living unit with its own cooking, sleeping and sanitation facilities, on the same lot as a single-family home. It can be internal, attached or detached, but it doesn't include a motorhome, camper, RV or "tiny home on wheels."

The section applies "only to cities with a population greater than ten thousand (10,000)." It doesn't mention counties. [1][3]

What covered cities must change by February 1, 2027

By February 1, 2027, each covered city must amend its comprehensive plan and land use rules for all land zoned for single-family homes (except historic districts and designated historic properties) to:

• allow either one internal ADU or one detached ADU per lot, in the rear yard or subject to the main home's setbacks;
• prohibit requiring off-street or guest parking, unless the main home has no off-street parking, the street isn't paved and built for on-street parking, or the home is within a quarter mile of transit, an employment area or commercial services;
• prohibit impact fees or utility connection fees for ADUs that are higher than for other single-family homes;
• prohibit limiting an ADU to "less than one thousand (1000) square feet or seventy-five percent (75%) of the size of the primary dwelling";
• prohibit owner-occupancy requirements for the house or the ADU;
• prohibit a height limit lower than the existing house;
• prohibit setback, lot size, coverage or frontage rules for ADUs that are stricter than for single-family homes in the same zone.

Until your city updates its code, its current ADU rules may be stricter than these. Ask your city planning office how it is applying the new law. [1]

How fast and how simply cities must approve

ADU projects that meet the city's land use requirements "shall be approved administratively and as a matter of right, without the need for discretionary approval." Approval standards must be "clear and objective" and must not discourage ADUs "through unreasonable cost or delay." The law sets no fixed number of days.

Cities can still apply building and fire codes, flood plain rules, water, sewer, storm drainage, road access and utility rules, environmental and critical habitat protections, and setbacks for rights-of-way and easements. They may also adopt rules that are less restrictive than the state's. [1]

HOAs and covenants

Under Idaho Code 55-3212, as amended by SB 1354, no homeowners' association may "add, amend, or enforce any covenant, condition, or restriction in such a way that limits or prohibits accessory dwelling units" unless the affected owner expressly agrees in writing.

An association may still adopt "reasonable rules" on ADUs allowed by law, including design consistent with the main home, size and height limits, setbacks, open space, parking controls and bedroom requirements. The section doesn't apply to short-term or vacation rentals of 30 days or less. [2][4]

Short-term rentals

Idaho Code 67-6539 says neither a county nor a city may enact or enforce an ordinance "that has the express or practical effect of prohibiting any type of short-term rentals." Local governments may adopt reasonable regulations to protect public health and safety, as long as they don't impose different restrictions on short-term rentals than on other single-family homes, except as the section permits. Check your city's registration or licensing rules before you list an ADU. [5]

Tiny homes on wheels and garage conversions

A tiny home on wheels isn't an ADU under the state definition, so the state ADU rules don't cover it. See our backyard tiny house guide.

The state law doesn't specifically address garage conversions. An internal or attached ADU is covered, so ask your city how it treats converting a garage. See our garage conversion guide. [1]

Where local rules differ

Smaller cities and counties set their own rules, and covered cities have until February 1, 2027 to update theirs. Boise, for example, currently caps ADUs at 900 square feet or 70% of the main home, whichever is smaller. Both numbers are lower than the figures in the state law, which says that by February 1, 2027 covered cities must amend their rules to prohibit limiting an ADU to "less than one thousand (1000) square feet or seventy-five percent (75%) of the size of the primary dwelling." Ask your city which limit it applies until then. See our Boise ADU guide.

For the general process, see our guides to the ADU permit process, ADU costs and ADU financing. [1][6]

Idaho laws to know

• SB 1354 (2026), Idaho Code 67-6541

  Bars cities over 10,000 people from banning ADUs in residential zones and requires code updates by February 1, 2027 with limits on size, parking, fees, height and owner-occupancy rules. [1][3]

• SB 1354 (2026), Idaho Code 55-3212

  Bars homeowners' associations from adding, amending or enforcing covenants that limit or prohibit ADUs without the owner's written agreement. [2][3]

• Idaho Code 67-6539, short-term rentals

  Bars cities and counties from prohibiting short-term rentals while allowing reasonable health and safety regulations. [5]

Questions people ask