ADUsUpdated 6 min readFloorGen team
Montana ADU Law (SB 528): 1,000 sq ft or 75% of the House
Montana ADU law explained: SB 528 and MCA 76-2-345, the 1,000 sq ft or 75% size rule, what cities cannot require, the 2025 county law, and the court case.
Montana ADU law requires every city and town with zoning to allow at least one accessory dwelling unit by right on any lot with a single-family home, attached, detached or inside the house. Under SB 528 (2023), now MCA 76-2-345, an attached or detached ADU may not be more than 75% of the house's gross floor area or 1,000 sq ft, whichever is less, and the city may not require extra parking, owner occupancy, matching design or impact fees. Since 2025 a parallel law covers zoned counties. Law current as of October 2026.
Below: the statute line by line, how the size rule works with real numbers, what happened in court, and an 800 sq ft plan checked against the formula. For the other states with ADU mandates, see ADU laws by state.
What the Montana ADU law says (MCA 76-2-345)
SB 528 was enacted as chapter 502, Laws of 2023, and codified in the municipal zoning part of Title 76. It applies to a "municipality": an incorporated city, town or consolidated city-county that exercises zoning powers. The 2025 edition of the MCA shows the section unchanged since 2023.
| Topic | Rule |
|---|---|
| Number | At least one ADU by right on a lot or parcel with a single-family dwelling [1(a)] |
| Type | Attached, detached or internal to the house [1(b)] |
| Size | An attached or detached ADU may not be more than 75% of the house's gross floor area or 1,000 sq ft, whichever is less [1(c)] |
| Parking | No added parking and no fees in lieu of parking [2(a)] |
| Design | No requirement to match the house's exterior design, roof pitch or finish materials [2(b)] |
| Owner occupancy | Not allowed for either the house or the ADU [2(c)] |
| Occupants | No familial, marital or employment relationship required [2(d)] |
| Fees | No impact fees [2(e)]; a one-time review fee of up to $250 per ADU, plus normal building fees [4] |
| Street improvements | Only to repair a street damaged by the construction [2(f)] |
| Height, setbacks, lot size, coverage, frontage | No more restrictive than for the house on the lot [2(g)] |
| Covenants | The city may not require a restrictive covenant on the ADU [2(i)] |
| Short-term rentals | The city may still regulate them [3] |
| Utilities | The city may require will-serve letters from the city water and sewer systems [7] |
| Deadline | Rules not adopted by January 1, 2024 are replaced by the statute; noncompliant rules in effect on or after that date are void [5] |
"By right" is defined in the statute: approval without a public hearing, a variance, a conditional use permit or other discretionary zoning action beyond checking that the site plan conforms to the zoning [9(b)]. The section does not override building codes, fire codes or public health rules [6], and a city may be more permissive than it [8].
The size rule with real numbers
"Gross floor area" in the statute means the interior habitable area of the house or the ADU [9(c)], so a garage, an unfinished basement or a crawl space should not count toward the house. The 75% branch decides for any house under about 1,333 sq ft of habitable area; above that, the 1,000 sq ft ceiling does.
| House habitable area | 75% of house | Maximum ADU |
|---|---|---|
| 800 sq ft | 600 sq ft | 600 sq ft |
| 1,000 sq ft | 750 sq ft | 750 sq ft |
| 1,200 sq ft | 900 sq ft | 900 sq ft |
| 1,333 sq ft | 1,000 sq ft | 1,000 sq ft |
| 1,800 sq ft | 1,350 sq ft | 1,000 sq ft |
The definition of an ADU adds one more condition: it must be on the same parcel as a single-family dwelling "of greater square footage" [9(a)]. With the 75% rule that is automatic for attached and detached units.
Counties: MCA 76-2-215 (2025)
SB 532 (chapter 619, Laws of 2025) added MCA 76-2-215 for counties that have adopted zoning. It also requires at least one ADU by right, attached, detached or internal, and bars owner-occupancy rules, matching design, relationship requirements, impact fees, and height, setback, lot size, coverage and frontage limits stricter than the house's. Its deadline was October 1, 2025. Two differences from the city section: the county section has no size formula and no parking clause, and it gives ADUs on parcels with existing public water and sewer a 15-day expedited state sanitation review (through September 30, 2029). Unzoned rural land is not covered by either section; septic and well rules still apply there.
The court challenge
A group called Montanans Against Irresponsible Densification sued over SB 528 and other 2023 housing laws. A Gallatin County judge blocked them with a preliminary injunction in December 2023; the Montana Supreme Court reversed that injunction in September 2024. On the merits, the district court upheld the housing laws in March 2025 except for one land-use planning law, and on March 17, 2026 the Supreme Court held that the planning law did not facially violate the right of public participation, affirmed that the laws did not violate equal protection, and vacated a ruling about private covenants. The Supreme Court's unanimous March 2026 decision left the ADU law in force, and as of October 2026 it applies statewide (MCA 76-2-345 for cities; MCA 76-2-215 for counties).
An 800 sq ft ADU against the formula
FloorGen's 800 sq ft 2-bed ADU template is a detached cottage 40 × 20 ft to the outside of its walls (800.1 sq ft). Measured to the inside of the exterior walls it is 736.3 sq ft, and its rooms add up to 700.5 sq ft net. Using the larger, inside-the-walls figure as the ADU's interior area, it fits the 75% rule next to a house with at least 982 sq ft of habitable area (736.3 ÷ 0.75), and it is under the 1,000 sq ft ceiling for any house.
| Measured to | Area |
|---|---|
| Outside faces of the exterior walls | 800.1 sq ft |
| Inside faces of the exterior walls | 736.3 sq ft |
| Sum of the room areas (net) | 700.5 sq ft |
How a city measures "interior habitable area" is its call, and some exclude closets, halls or laundry areas, which would bring the number down. For a smaller house, say 850 sq ft of habitable area (a 637.5 sq ft limit), drop the dining room into the living room and shorten the cottage by about 6 ft, or go to one bedroom.
Building code and winter
Montana adopts the International Residential Code with state amendments, and the statute leaves building, fire and health rules in force. A detached ADU needs habitable rooms of at least 70 sq ft and 7 ft wide (2021 IRC R304), an emergency escape opening in each bedroom (2021 IRC R310), and foundations below your area's frost depth, which can be several feet in much of the state. See legal bedroom requirements for escape-opening sizes and how to draw floor plans for a permit for the drawing set.
FloorGen's IRC profile checks room areas and widths, ceiling heights, bedroom escape windows, stairs and door swings; it does not check zoning, the 75% formula or frost depth. For comparison, Arizona's ADU law uses the same 1,000 sq ft or 75% numbers, but as the size a city must allow. Utah's SB 284 leaves size to cities but guarantees a detached ADU on lots of 11,000 sq ft or more, and Maine's ADU law sets only a 190 sq ft minimum.
More from the blog
ADUs6 min read
ADU Laws by State: Which States Require Cities to Allow ADUsADUs7 min read
Utah ADU Rules 2026: SB 284 and Detached ADUsADUs6 min read
Washington ADU Law (HB 1337): Two ADUs Per Lot Explained