ADU rules in Butte-Silver Bow

By right under state law, not under the local code. Butte-Silver Bow's zoning ordinance (B-SB MC Title 17, from Ord. 53 of 1978, codified through Ord. 2025-03 of 10-1-2025) contains no ADU provisions at all, and its R-1 and R-2 chapters still print the opposite rule: accessory structures 'shall not contain any habitable space or room' (17.10.020(D), 17.12.020(C)). Because the consolidated city-county never adopted ADU regulations, MCA 76-2-345(5) has controlled since 1-1-2024: the government must review and permit ADUs directly under the statute, and local regulations that conflict are void. So one detached backyard ADU is allowed by right on any lot containing a single-family dwelling, up to the state size line, with ADU setbacks, heights, and lot standards no stricter than the house's own (76-2-345(2)(g)).

By right under MCA 76-2-345(1)(b): the ADU 'may be attached, detached, or internal to the single-family dwelling'. Before the statute, the local code's only second-unit pathways were narrow: two-family dwellings are permitted uses in R-2, R-3, and R-C (17.12.020, 17.14.020, 17.20.020), and 17.38.110 lets a residence over 1,500 sq ft in an R-2 zone be converted to up to two units; R-1 and R1-S allowed only one single-family dwelling per lot. The state mandate overrides those limits for one ADU (attached, internal, or basement) on any single-family lot regardless of zone.

The local code sets no ADU size standard because it has no ADU category, so the statutory line in MCA 76-2-345(1)(c) governs: a detached or attached ADU 'may not be more than 75% of the gross floor area of the single-family dwelling or 1,000 square feet, whichever is less'. Gross floor area means interior habitable area (76-2-345(9)(c)), and the primary dwelling must be of greater square footage than the ADU (9)(a). The pending zoning rewrite draft carries the same 1,000 sq ft / 75% cap forward. A municipality may be more permissive than the statute (subsection (8)), but Butte-Silver Bow has not adopted anything more permissive.

State law forbids it: MCA 76-2-345(2)(c) bars requiring that either the house or the ADU be owner-occupied, and (2)(d) bars requiring any familial, marital, or employment relationship between the occupants of the two units. The local code has no owner-occupancy rule to void; Title 17 simply never regulated ADUs. Neither an at-application nor an ongoing owner-occupancy condition may be imposed.

MCA 76-2-345(2)(a) prohibits requiring additional parking, or fees in lieu of parking, to accommodate an ADU. Butte-Silver Bow's own parking table (17.40.900) requires two off-street spaces per single-family dwelling and 1.5 per multifamily unit but contains no ADU line, so the house's own two-space requirement stands while the ADU adds nothing. The pending rewrite draft states outright that off-street parking is not required for ADUs. Caveat: if Butte-Silver Bow's zoning were ever characterized as county zoning under Title 76 ch. 2 part 2 instead of municipal part 3, the county ADU statute (76-2-215) contains no parking prohibition, but even then no local ADU parking standard exists to apply.

By right as defined in MCA 76-2-345(9)(b): approval may not require a public hearing, a variance, conditional use permit, special permit or special exception, or any discretionary zoning action beyond a determination that the site plan conforms with applicable regulations. Butte-Silver Bow may charge a one-time ADU application fee up to $250 plus its normal building fees (76-2-345(4)) and may require will-serve letters from municipal water and sewer systems (76-2-345(7)). Practically, that means a building permit plus administrative zoning review through the Planning and Building departments; the code itself still has no ADU application track, so applications are processed directly under the statute.

Nothing in the municipal code restricts renting an ADU long-term. Butte-Silver Bow has no short-term-rental ordinance: Title 17 (zoning) and Title 5 (business licenses) contain no vacation-rental, tourist-home, or STR licensing scheme (the only 'short-term' string in Title 17 is about construction liability). MCA 76-2-345(3) expressly preserves local authority to regulate short-term rentals (defined at MCA 15-68-101 as stays under 30 days), but the city-county has not exercised it, and Montana has no statewide STR land-use preemption. No local rule today bars selling or subdividing an ADU separately, though the pending rewrite draft would prohibit separate sale.