ADU rules in Twin Falls

Twin Falls replaced its zoning code wholesale: the Uniform Development Code (Title 10) was adopted December 8, 2025, took effect January 1, 2026, and was amended by Ordinance 2026-006 (the city dates the amended text June 27, 2026). In the Table of Uses at section 10-3-2(K), the row 'Dwellings - Accessory Dwelling Unit (ADU)' carries a 'Z' in the AG, RR, TN-1, TN-2, TN-3 and AP districts, which is exactly the set of districts where a detached single dwelling unit is a permitted 'P' use. Section 10-6-10(B)(3)(b)(iv) lets the unit be 'located in a detached accessory building', and 10-6-10(B)(3)(a) allows one ADU on any legal lot or parcel that already holds a legal single household dwelling. A 'Z' use is not a hearing item: staff approve it through a Zoning Use Permit once the listed conditions are met.

The same standard covers every configuration: under 10-6-10(B)(3)(b)(iv) the ADU 'may be added to, included within the principal residence, or located in a detached accessory building', so an addition, a basement or internal conversion, and a backyard cottage are all treated identically. Subsection (b)(v)(B) adds that any existing structure converted into an ADU must meet all of the requirements of the section. Only one ADU is allowed per legal lot or parcel regardless of type. That is more generous than Idaho Code 67-6541(2)(a), which only obliges a covered city to allow either one internal unit or one detached unit.

Ordinance 2026-006 rewrote the size rule at 10-6-10(B)(3)(b)(i) to read 'seventy-five percent (75%) of the square footage of the living space for the principal residence or 1,000 sq. ft.; whichever is greater', so a modest primary home still supports a full 1,000 square foot unit. The same ordinance rewrote the height rule at (b)(ii)(A) to tie ADUs to 'the height limit listed for the principal residence as required for the base zoning district' instead of the 15 foot and 35 foot accessory-building tables in 10-6-10(A); those base limits are 35 feet in TN-1 and TN-2 (10-3-7(C)(2), 10-3-8(C)(2)) and 40 feet in AG, RR and TN-3 (10-3-4, 10-3-6, 10-3-9). Setbacks under (b)(iii) are the principal building's rather than the looser accessory-building setbacks, which in TN-1 means 15 feet front, 7 feet side and 15 feet rear. The code sets no separate minimum lot size, lot coverage cap or floor-area ratio for ADUs.

Nothing in the Uniform Development Code requires an owner to live on the property. Section 10-6-10(B)(3) sets out the complete list of ADU conditions (applicability, size, height, setbacks, types, design, parking, utilities) and none of them touches occupancy or ownership; a full-text search of the 116-page Title 10 returns no owner-occupancy language for any dwelling. Idaho Code 67-6541(2)(e), added by 2026 ch. 265 sec. 2 and effective July 1, 2026, separately requires covered cities to prohibit an owner-occupancy requirement for either unit by February 1, 2027, so Twin Falls is already aligned rather than merely scheduled for change. Note that the city's 'Bed & Breakfast Facility' use is defined as a 4 to 7 guest room lodging establishment that is 'Owner or operator occupied', but that is a distinct commercial use and does not reach ADUs.

Ordinance 2026-006 rewrote 10-6-10(B)(3)(b)(vi) so that the 'one additional parking space ... for exclusive use of the accessory dwelling unit' applies only where the principal dwelling has no off-street parking, or where the adjoining street does not provide on-street parking. The parking chapter defers to that rule: the residential block of the minimum-parking table at 10-8-4 lists 'Accessory Dwelling Unit: See Zoning Use Permit criteria of this Title' rather than the 2 spaces per dwelling unit that applies to other homes. Any space that is required must be hard surfaced in concrete, asphalt or an engineer-approved material under 10-8-1(B). Idaho Code 67-6541(2)(b) would also let the city require parking where the principal dwelling is within a quarter mile of transit, an employment area, or commercial services; Twin Falls did not adopt that third trigger, which 67-6541(5) permits because the local rule is the less restrictive one.

An ADU is a 'Z' use, which 10-3-2(I)(2) defines as a use that 'may be permitted in the subject district if reviewed and approved with conditions under specific provisions of this Title found in City Code 10-6: Zoning Use Permit Regulations'. That approval is a Zoning Use Permit, and Table 1 of 10-2-1 (Zoning Application Review & Procedures Chart) files it under 'Administrative Final Decisions' with City Staff as the decision maker, no public hearing, and only the Planning & Zoning Commission and City Council as appeal bodies. Section 10-2-2(G)(2) states that 'The Administrator shall authorize Zoning Use Permits upon demonstration that the conditions in City Code 10-6 ... have been met', and 10-2-2(A)(1) requires a determination within 15 business days of a complete application; the permit voids if compliance is not proven within one year. The city's Zoning Use Permit application (revision 04-2026) carries an 'Accessory Dwelling Unit $250' fee checkbox. A separate building permit and certificate of occupancy are still required under 10-2-2(B).

Neither Title 10 nor the rest of the Twin Falls City Code regulates rentals of an ADU: the city code on American Legal has no short-term-rental chapter, and the Uniform Development Code has no short-term-rental use category at all (the nearest lodging use, 'Bed & Breakfast Facility', is defined as 4 to 7 guest rooms that are owner or operator occupied, which a whole-unit rental is not). Idaho Code 67-6539, rewritten by 2026 ch. 22 sec. 1 effective July 1, 2026, bars any city ordinance with 'the express or practical effect of prohibiting any type of short-term rentals', classifies a short-term rental as a non-transient residential use for zoning and building code purposes, and in subsection (5) forbids requiring 'a license, fee, permit, certification, or registration to operate a short-term rental'. Generally applicable noise, parking, nuisance, curfew and traffic rules still apply under 67-6539(6). The ADU is not separable from the house: 10-6-10(B)(3)(b)(vii) requires it to take water, sewer and sanitation service from the primary dwelling with 'Additional service lines and/or meters shall not be installed', and the code has no provision for splitting an ADU onto its own lot.