ADU rules in Idaho Falls

A detached backyard cottage is a permitted use in every one of Idaho Falls's eight residential zones. Table 11-2-1 marks 'Dwelling, Accessory Unit' P (permitted use) in RE, RP, R1, R2, TN, RMH, R3 and R3A, and 11-2-6(N)(1)(c) expressly allows the unit to be built 'As a separate structure.' Only one accessory dwelling unit is allowed per lot, and it is not counted in density calculations (11-2-6(N)(5)). Placement today is governed by ADU-specific exceptions in 11-2-6(N)(2)(b): the side yard must match what a primary structure needs in that zone, and the rear setback is at least twelve feet unless the lot abuts a public or private alley at least sixteen feet wide, in which case Table 11-3-3(3) allows three feet; a detached unit taller than twelve feet must meet full primary-building setbacks under Table 11-3-3(1). Ordinance 3663, adopted June 25, 2026 with a delayed effective date of midnight January 31, 2027, deletes those two exceptions so that a detached ADU will simply follow the ordinary accessory-structure standards in Table 11-3-3.

Internal and attached units run on exactly the same permitted-use listing as detached ones: 11-2-6(N)(1) allows the accessory dwelling unit 'As an integral portion of a principal dwelling unit on any floor or basement' or 'As an attached structure to the principal dwelling unit,' so a basement conversion, an upstairs unit or a rear addition needs no different approval than a backyard cottage. Two design rules currently bite only on interior and attached units: 11-2-6(N)(2)(g) allows just one entrance on the front building elevation where the ADU is inside the primary structure (unless multiple front entrances already exist), and 11-2-6(N)(2)(h) requires a separate entrance, compliance with building code for a separate unit, and functional separation from the primary dwelling. Ordinance 3663 repeals both of those subsections effective January 31, 2027, leaving building-code separation as the only real constraint. In commercial and industrial zones (CC, LC, HC, PT, LM and I and M) the accessory unit is permitted only as a caretaker's residence built into a floor of the structure, and units in that category are exempt from all of the 11-2-6(N)(2) dimensional and design standards.

The operative cap is flat and small: 'The maximum size of the accessory dwelling is seven hundred and fifty (750) sq. ft.' (11-2-6(N)(2)(a)), with no percent-of-primary alternative and no per-zone variation. Only one accessory dwelling unit is permitted per lot (11-2-6(N)(5)). There is no ADU-specific height cap, but 11-2-6(N)(2)(c) makes a detached unit over twelve feet tall meet primary-building setbacks, and Table 11-3-3 caps accessory structures in the RP, R1, R2, TN and RMH zones at twelve feet with reduced setbacks or twenty-four feet with primary-building setbacks. Ordinance 3663, adopted June 25, 2026, replaces the 750 figure with 'one thousand (1000) sq ft or seventy-five percent (75%) of the size of the primary dwelling, whichever is greater,' matching the floor Idaho Code 67-6541(2)(d) sets for cities over 10,000, but the council deliberately delayed that ordinance to midnight January 31, 2027, so 750 sq ft is what a permit application faces today.

Idaho Falls still requires it, and enforces it against the title: 'The property owner shall occupy either the principal dwelling unit or the accessory dwelling unit as their primary residence, except for units accessory to nonresidential uses. This requirement shall be enforced through recordation of a deed restriction with the Bonneville County Recorder' (11-2-6(N)(4)). The city's ADU handout confirms the practical mechanics, that the deed restriction is filed by the homeowner as part of the building permit application. The requirement is ongoing rather than a one-time affidavit, since the recorded restriction runs with the land, and it means only one of the two units on the lot can be rented out. Idaho Code 67-6541(2)(e) forbids an owner-occupancy requirement for either the primary dwelling or the ADU and gives covered cities until February 1, 2027 to amend their codes; Idaho Falls adopted the repeal as part of Ordinance 3663 on June 25, 2026 but set it to take effect at the last possible moment, midnight January 31, 2027, so the deed restriction is still the operative local rule.

Section 11-2-6(N)(3) requires 'One (1) parking space ... on the lot where the accessory dwelling is located in addition to the existing minimum parking requirement for the principal dwelling unit,' and Table 11-4-2 lists 'Dwelling Unit, Accessory: 1.' A driveway counts toward that space, which keeps most infill lots workable, but converting the garage into the ADU is not permitted unless the principal dwelling's required parking can be provided elsewhere on the lot. Because a single-unit detached dwelling needs two spaces under Table 11-4-2, a house with an ADU currently needs three off-street spaces in total. Ordinance 3663 strikes both the 11-2-6(N)(3) sentence and the Table 11-4-2 row effective January 31, 2027, leaving no ADU parking requirement at all (the garage-conversion sentence survives); that is a flat repeal, more generous than Idaho Code 67-6541(2)(b), which would have let the city keep a requirement where the principal dwelling has no off-street parking, where the street is unpaved, or where the house is within a quarter mile of transit, an employment area or commercial services.

No hearing, no discretionary approval. Table 11-2-1 lists 'Dwelling, Accessory Unit' as P, which the table legend defines as a permitted use, in all eight residential zones, and the table uses separate C1, C2 and C3 codes for administrative, Board of Adjustment and City Council conditional uses that the ADU line does not carry. Table 11-6-1 (Summary of Actions/Decisions) contains no ADU entry, so the path is a building permit through the city's Cityworks portal with zoning-compliance review by staff, plus recordation of the owner-occupancy deed restriction at application until that requirement lapses. Idaho Code 67-6541(3), effective July 1, 2026, independently requires that conforming ADU projects 'be approved administratively and as a matter of right, without the need for discretionary approval,' which Idaho Falls already satisfied.

Long-term renting is unrestricted except through the owner-occupancy rule: because 11-2-6(N)(4) makes the owner live in the principal dwelling or the ADU, exactly one of the two units can be leased today, and both become rentable when Ordinance 3663 takes effect on February 1, 2027. Short-term rental is squarely allowed: 'Short Term Rental' is a P use in all eight residential zones and 11-2-6(AA)(1) says short-term rentals are 'Allowed in all zones where residential uses are also allowed,' barring only units in an accessory structure 'not been specifically designed for human habitation,' which an ADU by definition is. The city charges no short-term-rental license, permit or registration, and Idaho Code 67-6539(5), as rewritten effective July 1, 2026, now forbids one statewide along with day limits, caps, conditional use permits and owner-occupancy conditions; Idaho Falls's own Ordinance 3664 of June 25, 2026 already deleted its one-guest-party-at-a-time limit, and the city's general business registration in 4-17-2 reaches only businesses that need a Fire Code Permit, so it does not touch short-term rentals. What does not change is 11-2-6(N)(6): the accessory dwelling unit 'shall not be sold separately or converted to any form of legal ownership different from the principal dwelling unit,' a rule Ordinance 3663 keeps (renumbered to (N)(5)).