ADU rules in Missoula

Missoula abolished the ADU as a separate zoning category when its Unified Development Code (Title 22) took effect March 4, 2026. In its place, every parcel that allows residential use is permitted a minimum of two dwelling units by right regardless of parcel area, so a backyard cottage is simply a second one-unit house, a building type allowed in all seven residential districts (R-R1, LU-R1, LU-R2, U-R1 through U-R4) and in every mixed-use district. Detached ADUs built under the old Title 20 are reclassified as one-unit houses. The second house is a principal building, so district setbacks (10 to 15 ft front, 5 ft interior side and rear in most districts) and the district height limit apply rather than accessory-structure setbacks. When two one-unit houses share a parcel, the City Engineer may let them share a utility line by classifying one as an accessory structure for utility purposes only (UDC 4.12.01-D).

An attached or internal second unit (basement apartment, addition, or internal conversion) is reviewed as the duplex building type, defined as a standalone primary structure consisting of two side-by-side or stacked dwelling units; duplexes are allowed in every residential district including rural R-R1. Structures that contained a primary dwelling with an attached ADU before March 4, 2026 are deemed duplexes by the UDC's transition rules. Legally established non-residential buildings may also be converted to dwelling units.

The UDC sets no ADU-style size cap; the old Title 20 limits died with that title. Size is governed by floor area ratio: on a two-unit parcel the combined gross floor area of all buildings may not exceed 0.5 times the parcel area in LU-R1, LU-R2, and U-R1, or 0.6 times in U-R2, U-R3, and U-R4, with basements, garages, and accessory structures excluded from the calculation, and one-unit house and duplex development on parcels of 4,000 sq ft or less exempt from FAR entirely. Rural R-R1 uses density (1 unit per 15,000 sq ft) instead of FAR, but the two-units-by-right floor applies on any parcel regardless of area. Maximum building height is 35 ft in most residential districts (40 ft in LU-R2, 45 ft in U-R4). This is more permissive than the statewide floor in MCA 76-2-345(1)(c) (ADUs up to the lesser of 75 percent of the primary dwelling or 1,000 sq ft), which subsection (8) expressly allows.

Neither unit must be occupied by the owner: the UDC contains no owner-occupancy requirement for dwellings, and the two-dwelling-unit entitlement carries no residency condition. Owner occupancy appears only in the definitions of bed-and-breakfast and Type 3 short-term rentals. The Type 1 short-term-rental (tourist home) rules expressly contemplate a non-owner-occupied additional unit on the same parcel as the host's primary residence. Montana law reinforces this: MCA 76-2-345(2)(c), in force since January 1, 2024, barred municipalities from requiring that either the single-family dwelling or the ADU be owner-occupied.

Missoula dropped residential parking minimums citywide with the UDC: no minimum vehicle parking ratios for dwellings appear anywhere in Title 22. Article 6.3 regulates only the design and construction of driveways and parking an owner chooses to build (for one-unit houses and duplexes: garage doors set back 20 ft from the sidewalk, driveways capped at 25 ft wide and 40 percent of the front yard). One long-term bicycle storage space per dwelling unit is required unless the unit has a private garage. MCA 76-2-345(2)(a) has independently barred ADU parking mandates statewide since January 2024.

A second dwelling unit needs only zoning compliance review of the building permit or a zoning compliance permit, decided administratively by the Planning Administrator; the UDC states that zoning compliance permits and other ministerial permits may be issued without any further discretionary review or analysis, and decisions turn solely on whether the proposal complies with applicable requirements. The UDC contains no conditional use permit process at all: uses and building types are either allowed or not allowed in each district. A Historic Preservation Permit can add a review layer inside Historic Resource Overlay Districts.

Long-term rental is unrestricted: household living covers rented dwelling units on month-to-month or longer tenancy wherever residential use is allowed. Renting a second unit for stays under 28 consecutive days is a Type 1 tourist home, an allowed use in all residential districts, but it requires annual registration under Title 5 (2025 fee $636, renewals due January 31 per the city's STR page), neighbor notification, an occupancy cap of one paying guest per 150 sq ft of floor area, a 24/7 responsible party, and no signs; no more than one Type 1 or Type 2 short-term rental is permitted per parcel in residential districts, and any city-ordinance violation at the STR revokes the registration at that location for one year. Duplex, rowhouse, and cottage court units may be platted with each dwelling on its own lot, and townhome-exemption and condominium processes allow separate ownership of units.