ADU rules in Helena

Ordinance 3329 (finally passed April 6, 2026, effective May 5, 2026), Helena's Montana Land Use Planning Act targeted-amendments ordinance, 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)). Before that ordinance Helena's code had no ADU provisions at all, and from January 1, 2024 ADUs were reviewed directly under the state mandate in MCA 76-2-345. Because Ord. 3329 and its companion subdivision ordinance completed Helena's move to the Land Use Planning Act, the by-right allowance now rests on the city's own code rather than on that statute.

Attached and internal ADUs fall under the same by-right land use listing as detached ones. HCC 11-2-5(J)(3) sets separate size 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. 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. 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 or setback; the district's normal dimensional standards in HCC 11-4-2 apply (for example a 30-foot height maximum and 10-foot front and rear setbacks in R-1/R-2). 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 (HCC 11-2-5(J) and the ADU definition in 11-2-4) contain no owner-occupancy requirement, at application or ongoing; a full-text search of the City Code on 2026-08-04 found 'owner-occupied' only in the bed-and-breakfast definition, and the code has not been re-codified since. Helena has completed its Montana Land Use Planning Act transition, so the statewide owner-occupancy ban in MCA 76-2-345(2)(c) no longer binds it directly (MCA 76-25-105(4)); the absence of any requirement now rests on the city's own code.

The off-street parking table lists 'Residence, accessory dwelling unit: None required' (HCC 11-22-3, as amended by Ord. 3329); the principal single-dwelling residence needs 1 space. In the Downtown and Transitional Residential districts parking is instead governed by HCC chapter 9, whose standards address parking setbacks but set no minimum number of spaces. State law independently backs the ADU result: since October 1, 2026, MCA 76-25-303(1)(j) (HB 492, Laws of Montana 2025, ch. 682) bars a Land Use Planning Act city from requiring 'any minimum parking requirement for ... residential units under 1,200 square feet', and a Helena ADU is capped at 1,000 square feet unless it is an internal unit.

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 development and to multi-unit residential developments of three or more units, so an ADU needs only a standard building permit, applied for through the city's Civic Access portal. In the OSR district all construction additionally requires an administrative development permit (HCC 11-2-5(A)).

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' returned nothing on 2026-08-04, the code has not been re-codified since, and the City Commission adopted no rental ordinance through October 19, 2026. 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. Montana has no statewide preemption of local short-term-rental rules, so a future city STR ordinance remains possible.