ADU rules in Helena

Ordinance 3329 (finally passed April 6, 2026, part of Helena's Montana Land Use Planning Act update) added 'Residence, accessory dwelling unit (ADU)' to the land use table as a use permitted by right in every district that allows a single-dwelling residence: OSR, R-U, R-1/R-2, R-3, R-4/R-O, B-1, B-2, B-3, DT, TR, and CLM. An ADU is allowed accessory to a single-dwelling unit residence only, and one ADU is allowed per lot (HCC 11-2-5(J)). This implements MCA 76-2-345 (SB 528, 2023), which requires every Montana municipality to allow at least one attached, detached, or internal ADU by right on any lot with a single-family dwelling; from January 1, 2024 until the 2026 ordinance, Helena's code had no ADU provisions at all and ADUs were reviewed directly under the statute per MCA 76-2-345(5).

Attached and internal ADUs fall under the same by-right land use listing as detached ones. HCC 11-2-5(J)(3) sets separate bulk standards for detached, attached, and internal ADUs, and HCC 11-2-4 defines an ADU as a self-contained living unit on the same parcel as a single-dwelling unit of greater square footage with its own cooking, sleeping, and sanitation facilities, matching the definition in MCA 76-2-345(9)(a). Basement and attic conversions are internal ADUs, which may occupy the entirety of a basement or attic space.

HCC 11-2-5(J)(3) caps a detached or attached ADU at seventy-five percent of the square footage of the principal dwelling or 1,000 square feet, whichever is less, mirroring the statewide cap in MCA 76-2-345(1)(c). An internal ADU may instead occupy up to forty percent of the principal building or the entirety of a basement or attic. There is no ADU-specific height limit; the district's normal dimensional standards in HCC 11-4-2 apply (for example 30 feet in R-1/R-2), and MCA 76-2-345(2)(g) bars the city from holding ADUs to stricter height, setback, or lot standards than the single-family dwelling. Because the ADU definition requires the principal dwelling to be of greater square footage, the main house must always be larger than the ADU.

Helena's ADU provisions contain no owner-occupancy requirement, at application or ongoing; a full-text search of the entire City Code finds 'owner-occupied' only in the bed-and-breakfast definition. State law forecloses one in any case: MCA 76-2-345(2)(c) provides that a municipality may not require that the single-family dwelling or the accessory dwelling unit be occupied by the owner, and under 76-2-345(5) any non-compliant local ADU regulation is void.

The off-street parking table lists 'Residence, accessory dwelling unit: None required' (HCC 11-22-3, amended by the same Ord. 3329; the principal single-dwelling residence itself needs only 1 space). In the Downtown and Transitional Residential districts, parking is instead governed by HCC chapter 9, which sets no parking minimums at all. This matches MCA 76-2-345(2)(a), which prohibits municipalities from requiring additional parking, or fees in lieu, to accommodate an ADU.

An ADU is a 'P' use in the land use table, which HCC 11-2-3 defines as permitted in the district by right, consistent with applicable development standards: no conditional use permit and no public hearing. Helena's Commercial Site Review (HCC 11-27-1) applies only to commercial projects and residential buildings of three or more units, so an ADU needs a standard building permit, applied for through the city's Civic Access portal. MCA 76-2-345(9)(b) defines by right to exclude hearings, variances, and any discretionary action beyond site-plan conformance review, and the statute caps any ADU application fee at $250 plus normal building fees.

Nothing in the zoning title restricts renting an ADU long-term, and Helena has no short-term-rental regime: full-text searches of the City Code for 'short-term rental', 'vacation rental', and 'tourist home' return nothing (2026-08-04). MCA 76-2-345(3) expressly preserves the city's power to regulate short-term rentals of ADUs, so a future STR ordinance is possible, but none exists today. The ADU definition ties the unit to the same parcel as the principal dwelling and no provision allows separate sale. Bed and breakfasts are a separately defined owner-occupied use needing a conditional use permit in the lower-density residential districts (OSR, R-U, R-1/R-2).