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. 2026-01 of 6-17-2026) 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)). A full zoning rewrite that would add a local ADU section is still a draft before the Planning Board as of October 2026.

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). A municipality may be more permissive than the statute (subsection (8)), but Butte-Silver Bow has not adopted anything more permissive. Watch the pending rewrite: its third and fourth working drafts (August and October 2026) cut the cap to '500 square feet or 50% the gross floor area of the principal structure, whichever is smaller', stricter than the statutory line; that text is not law, and whether a local cap below the statute's line would survive 76-2-345(2)(h), which bars 'more onerous development standards' on ADUs, is untested.

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, so the ADU itself needs no space. Butte-Silver Bow's printed parking table (17.40.900) still demands 'two parking spaces per dwelling unit' for a single-family dwelling and has no ADU line, but since 10-1-2026 state law caps parking for the house too: amended MCA 76-2-304(5) (HB 492, Laws of Montana 2025, ch. 682) says zoning 'may not include provisions that require ... more than one parking space for each residential dwelling unit' or 'more than one-half parking space for each residential unit under 1,200 square feet'. So the house can be required to have at most one space (half a space if it is under 1,200 sq ft), and owners may still choose to build more (76-2-304(6)); the local table has not been amended to match. The pending rewrite drafts state that 'Off-street parking is not required for accessory dwelling units.' Caveat: both statutes sit in Title 76 ch. 2 part 3 (municipal zoning); if Butte-Silver Bow's zoning were characterized as county part 2 zoning, the county ADU statute (76-2-215) has no parking prohibition and HB 492 does not reach part 2, though 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. The pending rewrite drafts would change that if adopted: they add STR hosting permits, with Type 1 (owner present on the parcel, which covers renting an ADU while living in the house) and Type 2 (whole dwelling, not owner-occupied, barred in the Detached Residential district within 350 feet of another Type 2), and would prohibit selling or subdividing an ADU separately. None of that is in force today.