ADU rules in Bozeman

Bozeman repealed and replaced its entire Chapter 38 Unified Development Code by Ordinance 2025-12 (No. 2151, adopted 12-16-2025, effective 2-1-2026) under the Montana Land Use Planning Act. In the new code an ADU, defined as 'an accessory residential building, either attached or detached, which contains a dwelling unit', is a permitted accessory use with standards (A*) in every residential district (R-A, R-B, R-C, R-D) plus REMU, NEHMU, M-1 and M-2; each residential district's intent statement lists ADUs among the housing types the district is meant to accommodate. One ADU is allowed per lot. The one bite: detached ADUs (including second-story additions on detached garages) 'may be approved only if found compatible and consistent with the existing character and fabric of the neighborhood', an administrative design-compatibility finding that attached ADUs do not face. A detached ADU may not exceed 25 feet in height but may be taller than the principal dwelling.

The ADU definition covers 'either attached or detached' accessory residential buildings, and the standards allow ADUs above garages; the Planned Development Zone provisions likewise describe ADUs 'either within the primary building or in a freestanding accessory building', so internal conversions count as the attached form. Attached and internal ADUs follow the same Table 38.320.120.B-1 standards (one per lot, size cap, no deviations) but are not subject to the neighborhood-compatibility design finding that applies only to detached ADUs. 'The addition of an ADU on an infill lot does not require full compliance with the density standards of the base zoning district' (sec. 38.260.030).

Table 38.320.120.B-1: 'In no case may an ADU be larger than 1,000 square feet or 75 percent of the principal unit, whichever is larger', measured as the sum of habitable and occupiable spaces under the adopted building code. The larger-of formula is deliberately more generous than the line MCA 76-2-345(1)(c) draws for cities still zoning under Title 76 ch. 2 (75% or 1,000 sq ft, 'whichever is less'); on a 2,400 sq ft house Bozeman allows an 1,800 sq ft ADU. Height: a detached ADU may exceed the principal building's height but not 25 feet, and no accessory building may exceed the footprint of the principal building (sec. 38.320.120.A.5).

The ADU standards table has no occupancy or ownership row, and the only owner-occupancy rule anywhere in Chapter 38 is the bed-and-breakfast definition ('remains owner-occupied at all times'); nothing requires the owner to live in either unit, and no familial relationship is required. While Bozeman zoned under MCA Title 76 ch. 2, state law (76-2-345(2)(c)) barred owner-occupancy mandates outright; the city's MLUPA-based code continues without one even though that bar no longer directly applies (76-25-105(4)). The one occupancy tie that remains is for short-term rental only: renting the ADU as an STR requires the host's primary residence on the same lot (Type-2B).

Table 38.320.120.B-1: 'ADUs are not subject to minimum parking requirements. If parking is provided it must conform to all applicable standards.' State law now points the same way: since 10-1-2026 MCA 76-25-303(1)(j), as amended by HB 492 (Laws of Montana 2025, ch. 682), bars a MLUPA city from requiring any parking minimum for 'residential units under 1,200 square feet' or more than one space per dwelling unit. The catch is garage conversions: 'Garages may not be converted for use as ADUs unless all required parking for all uses on the lot is otherwise provided prior to conversion', though building an ADU above a garage is fine. Under the parking table in force since 10-1-2026 (sec. 38.530.040.C, Table 38.530.040-1), a house under 1,200 sq ft needs no parking and one of 1,200 sq ft or more needs one space, so that gate now bites only on larger houses.

ADUs are an accessory use ('A', not 'S' special use) in the allowed use table, and 'Accessory dwelling units' appear on the sketch plan list (sec. 38.740.060.A.3.d): a staff-level review that 'may be conducted as part of building permit review or as an independent application'. Sketch plans go to the community development department 'for a determination of compliance with the requirements of this chapter. Once compliance is achieved, the application will be approved for construction'; no public hearing or conditional use permit is involved. Two frictions: detached ADUs need the review authority's neighborhood-compatibility finding (Table 38.320.120.B-1), and inside the neighborhood conservation district a certificate of appropriateness is required for accessory structures unless exempt (sec. 38.320.120.A.7).

Nothing restricts renting an ADU on ordinary leases, and no owner-occupancy or familial tie applies. Short-term rental (under 28 consecutive days) requires a city hosting permit: an ADU STR is classified Type-2B, 'a short term rental of an accessory dwelling unit (ADU) on the same lot as the host's primary residence', so an owner who lives elsewhere cannot STR the ADU. Type 2 STRs (which include Type-2B) are an accessory use in R-A, R-B, R-C, R-D, REMU, NEHMU and most business districts, but not in M-1 or M-2, where ADUs are otherwise allowed. Ordinance 2026-06 (final adoption 7-28-2026, effective 8-27-2026) added Type 2 STRs to the R-A district, which the 2026 code had left at Type 1 only, so ADU STRs are now available in R-A too (the city's STR web page still says only Type 1 is allowed there). Dedicated non-primary-residence STRs (old Type 3) have been banned for new permits since Ordinance 2149 (December 2023); earlier permitted Type 3s survive only as legacy permits.