ADU rules in Boise

Boise's December 2023 zoning rewrite (Boise Development Code, City Code Title 11) lists Accessory Dwelling Unit as an allowed accessory use (A, never conditional) in every base residential district (R-1A Large Lot, R-1B Suburban, R-1C Traditional, R-2 Compact, R-3 Urban), in the MX-1, MX-2, MX-3, and MX-U mixed-use districts, and in the A-1 and A-2 open land districts. Construction of a new detached ADU is an enumerated creation method, and since Ord. 11-26 (passed 4-14-2026) a lot may contain up to two accessory residential units in any combination of ADUs and tiny homes on wheels. The ADU must be accessory to a single-family dwelling, meet the underlying district's dimensional standards (with the accessory-structure exceptions in Section 11-04-03.3), and may not exceed two stories.

Attached and internal ADUs are expressly allowed: the creation methods include internal conversion of an existing living area, basement, or attic; an addition to the principal dwelling; an addition to a detached accessory structure; and a new single-family home built with an internal ADU. The same 900 sq ft / 70 percent size cap and two-bedroom limit apply as for detached units. Converting an existing detached accessory structure or attached garage is also allowed provided it does not reduce off-street parking below the minimum required for the lot.

Section 11-03-03.2.B(4) caps an ADU at 900 square feet or 70 percent of the total floor area of the primary building, whichever is smaller, with no more than two bedrooms, and subsection (5) caps height at two stories. This cap now sits below the state floor: Idaho Code 67-6541(2)(d) (added 2026, effective 7-1-2026) requires cities over 10,000 to prohibit limiting ADU size to less than 1,000 square feet or 75 percent of the primary dwelling, with local amendments due by February 1, 2027. Until Boise amends, the 900 sq ft / 70 percent text remains the operative standard applied to applications.

The current ADU standards contain no owner-occupancy requirement for either unit; the full use-specific standards list (11-03-03.2.B(1)-(9)) was read and contains none, and no such requirement appears in the definitions, parking, or procedures chapters. Boise's pre-2023 code required proof of owner occupancy at application and ongoing owner occupancy of the primary or accessory unit enforced by a recorded deed restriction (legacy § 11-06-03.1.A(6)-(7)); the Boise Development Code effective 12-1-2023 eliminated that regime, so Boise is already conformant with Idaho Code 67-6541(2)(e), which requires covered cities to prohibit owner-occupancy requirements by 2-1-2027. Properties permitted under the old regime may still carry deed restrictions recorded in that era.

Table 11-04.9 (Off-Street Parking Requirements) leaves both the minimum and maximum cells of the Accessory Dwelling Unit row blank, and the table legend states a blank cell means no minimum or maximum requirement, so Boise requires no additional off-street parking for an ADU citywide. The only parking-related condition is in the creation standards: a garage or accessory-structure conversion may not reduce off-street parking below the minimum required for the lot (the primary dwelling's own requirement, generally 1 space per dwelling unit). This is already more permissive than the limited parking allowances preserved by Idaho Code 67-6541(2)(b).

Table 11-03.1 marks ADUs 'A*' (allowed use subject to use-specific standards), never 'C' (conditional use), in every district where they are allowed, so no public hearing or conditional use permit is involved. The city's process is a staff-level administrative planning review of a PLN application filed through the Accela portal, followed by a standard building permit; under Section 11-05-02.3, procedures not listed in the summary table are handled as Type 1 ministerial Zoning Compliance Review or Zoning Certificate decisions by the Planning Director. This aligns with Idaho Code 67-6541(3) (effective 7-1-2026), which requires conforming ADU projects to be approved administratively and as a matter of right.

Nothing in the Development Code restricts renting an ADU long term, and the city's ADU materials describe rentals as an intended use. For short-term rentals, Boise repealed its STR licensing chapter (Title 3, Chapter 22, created by Ord. 7-22 in March 2022) by Ord. 14-26, passed 5-12-2026 and effective 5-18-2026; the city clerk's page states a Boise short-term rental license is no longer required. The repeal anticipated Idaho Code 67-6539 as rewritten effective 7-1-2026, which bars any city license, fee, permit, certification, or registration for STRs and forbids STR-specific restrictions (owner-occupancy ties, day limits, caps, conditional use permits, and more); only the statute's short safety list plus generally applicable nuisance, noise, and parking rules remain enforceable. One quirk survives on the books: the tiny-home-on-wheels standards still say a THOW shall not be used as a short-term rental, citing the now-repealed Title 3 Chapter 22 definition; that clause applies to THOWs, not ADUs, and its enforceability under 67-6539 is doubtful.