ADU rules in Great Falls

A detached backyard ADU is allowed by right on any Great Falls lot that already has a single-family dwelling, but the rule is state law, not city code. Title 17, the Land Development Code, contains no accessory dwelling unit provision at all; because Great Falls never adopted compliant ADU regulations by January 1, 2024, MCA 76-2-345(5) requires the City to review and permit ADUs directly under the statute. The City says so itself on its Accessory Dwelling Units and Duplexes page, and it runs a dedicated ADU Application through the Building Division with an explicit DETACHED option. Do not confuse this with the code's 'accessory living space' (OCCGF 17.20.7.010), a separate and much narrower use that may not contain a stove or be rented.

Attached and internal ADUs are allowed on the same by-right footing as detached ones. MCA 76-2-345(1)(b) lets the unit be attached, detached, or internal to the single-family dwelling, and the City's own handout repeats that an ADU may be attached to the primary dwelling, separate from it, or inside it. The City's ADU Application form offers ATTACHED and DETACHED checkboxes; an internal conversion (a basement or upper-floor unit) is still covered by the statute even though the form does not name it, so expect to describe it in the scope-of-work line.

Great Falls applies the statutory ceiling verbatim because it has adopted no cap of its own: MCA 76-2-345(1)(c) limits an attached or detached ADU to no more than 75% of the gross floor area of the single-family dwelling or 1,000 square feet, whichever is less, and the City's handout restates that number. 'Gross floor area' means interior habitable area (76-2-345(9)(c)), and the statutory definition in 76-2-345(9)(a) requires the primary dwelling to be of greater square footage than the ADU. On height and setbacks the City is more permissive than the statute requires: its handout says ADUs get the same setbacks as an accessory structure, which in R-1, R-2 and R-3 means 5 feet at the side and rear rather than the 15, 8 or 6 foot side setbacks that apply to the house (Exhibit 20-4).

No owner-occupancy requirement applies, and the City could not impose one. MCA 76-2-345(2)(c) bars a municipality from requiring that either the single-family dwelling or the ADU be owner-occupied, and 76-2-345(2)(d) separately bars any familial, marital, or employment relationship test between the households. Great Falls has no ADU ordinance to conflict with this, and its own handout lists both prohibitions. Note the contrast with OCCGF 17.20.7.010(D)(3), which limits an 'accessory living space' to the occupant of the principal home, that person's immediate family, or 30-day guests; that restriction belongs to a different, non-dwelling use and does not reach a statutory ADU.

No additional off-street parking may be required for an ADU. MCA 76-2-345(2)(a) forbids a municipality from requiring extra parking to accommodate an ADU or charging fees in lieu, and the City's handout lists that prohibition first among the limits on city rules. The Land Development Code's own parking table (Chapter 36) still requires 2 spaces per single-family dwelling and 1.5 per multi-family unit, but that schedule cannot be extended to the ADU. Separately, the 2025 amendments to MCA 76-2-304 (effective October 1, 2026) will cap what the City can require for the principal dwelling at one space per dwelling unit, and at one-half space for residential units under 1,200 square feet.

Review is administrative. 'By right' is defined at MCA 76-2-345(9)(b) to exclude a public hearing, a variance, a conditional or special use permit, and any discretionary zoning action other than confirming that a site plan conforms with the zoning regulations. In practice the Building Division issues a dedicated ADU Application (form effective 3/2026) that collects sign-offs from NorthWestern Energy, Public Works Engineering and Environmental, Great Falls Fire Rescue, Addressing, Great Falls Utilities, the Floodplain Administrator, and the Planning and Community Development Director, then routes to zoning review and a building permit. The utility sign-offs are the will-serve step the statute permits at 76-2-345(7). The statute caps the one-time ADU application fee at $250 plus normal building fees; the City's posted residential zoning permit fee is $200.

An ADU may be rented long term: with owner occupancy and relationship tests both barred by MCA 76-2-345(2)(c) and (2)(d), and no city ADU ordinance in force, nothing in Great Falls conditions occupancy on who owns or lives in the house. MCA 76-2-345(3) expressly preserves a municipality's power to regulate short-term rentals as defined in 15-68-101, but a read of OCCGF Title 5 (business licenses and permits) and Title 17 Chapter 20 found no short-term-rental permit, registration, or zoning regime; the only related use is the bed and breakfast standard at 17.20.7.030, which requires the operator to live in the single-family dwelling and caps guest rooms at six. The ADU is not separately sellable as its own parcel without a subdivision, since the statute defines it as a unit on the same parcel as the single-family dwelling.