ADU rules in Caldwell

Detached ADUs are a permitted use by right in every residential district (RS-1 Semi-Rural, RS-2 Semi-Rural, R-1 Neighborhood, R-2 Community and R-3 Urban Residential), listed as 'P' in the 10-05-03 land use schedule. Code section 10-12-03-1(4)(B) states plainly that ADUs may be attached or detached, and (4)(C) caps the lot at one ADU. The table in 10-12-03-1(5) puts a detached ADU in the rear yard or an interior side yard, and in a street-side yard only when it sits behind a solid sight-obscuring fence at least 6 feet tall; a front-yard ADU is not allowed. Detached ADUs must keep at least a 3 foot setback from any interior or rear lot line under 10-12-03-1(2)(C)(6), and every table footnote conditions the use on there being an existing legal single-family detached home on the lot.

The same section allows an attached or internal ADU: 10-12-03-1(4)(B) permits ADUs 'attached or detached to the principal residential structure,' and the 10-01-10 definition describes an ADU as a secondary unit with its own sleeping, cooking and sanitation facilities. The catch for a basement or interior conversion is 10-12-03-1(4)(E): the ADU must have its own separate exterior entrance, or an entrance onto an internal common area that opens to the outside, and no internal access between the ADU and the principal residence is permitted. Attached ADUs follow the base zoning district setbacks instead of the accessory-structure setbacks (4)(J), and they are exempt from the yard-location table (4)(K). Only one ADU is allowed per parcel whether it is attached or detached.

Section 10-12-03-1(4)(D) limits an ADU to 900 square feet or 75 percent of the principal residence, whichever is less, and to no more than two bedrooms, excluding garages and other uninhabitable space. Height is not governed by the 22 foot and 33 foot detached-accessory-structure caps: 10-12-03-1(2)(D) sends ADUs to the district height schedule, which allows 45 feet in RS-1 and RS-2, 30 feet in R-1, 40 feet in R-2 and 45 feet in R-3. A separate accessory-structure rule, 10-12-03-1(2)(G), bars an accessory structure in a residential zone from exceeding the square footage of the principal residence unless a special use permit is granted. The 900 square foot cap is below the floor Idaho set in 2026: Idaho Code 67-6541(2)(d) requires covered cities to prohibit limiting an ADU to less than 1,000 square feet or 75 percent of the primary dwelling, and Caldwell has until February 1, 2027 to conform, so expect this number to rise.

Caldwell imposes no owner-occupancy condition on either unit. The ADU standards at 10-12-03-1(4) run from subsection A through subsection L and say nothing about who must live on the property, and an exact-phrase search of the entire Caldwell City Code turns up owner-occupancy language only in unrelated definitions (bed and breakfast inns and apartment ownership). State law now backstops that result: Idaho Code 67-6541(2)(e) requires covered cities to prohibit an owner-occupancy requirement for the primary dwelling or the ADU by February 1, 2027, so Caldwell has nothing to repeal.

Table 2 in the off-street parking standards sets the Accessory Dwelling Unit minimum at 0 parking spaces and the maximum at 1.0 per dwelling unit, so no additional off-street parking is required for an ADU and no more than one space may be provided for it. The principal dwelling keeps its own requirement of 2 spaces per unit for single-family and two-family homes. The one place parking bites is the Steunenberg Residential Historic District: 10-12-03-1(4)(L)(2) allows converting a detached garage into an ADU only if all off-street parking required by 10-02-05 is still maintained. Caldwell already sits inside the state floor here, since Idaho Code 67-6541(2)(b) requires covered cities to prohibit ADU parking mandates except in narrow situations.

In every residential district the land use schedule marks Dwelling - Accessory Dwelling Unit as 'P', which 10-05-03(2)(A) defines as allowed by right, so an ADU is approved through a building permit and zoning compliance review with no public hearing. The same by-right treatment applies in the T-N Downtown Transition Neighborhood and D-CB Downtown Central Business zones and in the C-D College, H-D Healthcare and P-D Public districts. The exceptions are the C-1 Neighborhood Commercial and C-2 Community Commercial zones, where 10-06-03 lists ADUs as 'SUP' and a special use permit means a neighborhood meeting and a commission or hearing examiner hearing. Design review does not normally apply to a single-family lot, though an ADU inside the UD-1 Downtown Urban Design overlay needs administrative staff-level design review, and one inside the Steunenberg historic district needs a Certificate of Appropriateness before any other permit issues.

Nothing in Chapter 10 restricts renting an ADU, sets a minimum lease term, or ties the unit to a family member or caretaker, and there is no owner-occupancy condition. The one ownership limit is 10-12-03-1(4)(I): the ADU shall not be subdivided from the main parcel it sits on, so it cannot be sold off separately. Caldwell has no short-term-rental ordinance at all: exact-phrase searches of the full city code for 'short term rental,' 'short-term rental' and 'vacation rental' return zero results, and no STR item appears in the city's 2026 council records. Idaho Code 67-6539, rewritten effective July 1, 2026, would preempt one anyway: it bars any city from prohibiting short-term rentals, from requiring a license, fee, permit, certification or registration to operate one, and from imposing owner-occupancy, day limits, numeric caps, extra parking or a conditional use permit in a residential zone.