ADU rules in Flathead County

A detached backyard ADU is a permitted use in every Flathead County zoning district that allows a single-family dwelling, and the county's definition expressly reaches a unit "detached from the principal single-family dwelling" (FCZR 8.05.160). Resolution 955IR, adopted 11-13-2025 to absorb the 2025 legislative session, moved ADUs out of the conditional-use lists in the R zones and into the permitted-use lists, leaving at most an administrative site plan review. One ADU is allowed per tract of record, it is held to the same maximum height and setbacks as the house (FCZR 5.01.030(6)), and it is one of the few accessory structures the county will let you place in a front yard (FCZR 5.01.030(2)(D)). MCA 76-2-215, binding on Montana counties since 10-1-2025, independently requires at least one ADU by right on any lot containing a single-family dwelling.

The same definition covers a unit "added to, created within, or detached from the principal single-family dwelling," so a basement conversion, an internal unit, or a room addition all qualify as the one ADU allowed per tract of record (FCZR 8.05.160). The district use lists do not distinguish attached from detached: a single line, "Dwelling unit, accessory (ADU)," appears in each permitted-use list, and MCA 76-2-215(1)(b) confirms an ADU "may be attached, detached, or internal to the single-family dwelling." For an attached or internal ADU, habitable space is measured from interior dimensions rather than exterior walls, which is slightly more generous than for a new detached building. Do not confuse this with the county's separate "accessory apartment" use (FCZR 8.02.020), a unit in the principal structure on business and industrial lots that is often a conditional use.

FCZR 8.05.160 caps an ADU at "1,000 square feet of habitable space or forty percent (40%) of habitable space of the principal single-family dwelling, whichever is greater." The "whichever is greater" wording is unusually generous for a size cap: a 4,000 sq ft house supports a 1,600 sq ft ADU, while every lot gets at least the flat 1,000 sq ft floor. A new detached ADU is measured from exterior walls; space added inside an existing structure is measured from interior dimensions, and garage area does not count as habitable space if the garage door is at least eight feet wide. There is no separate ADU height or setback standard: FCZR 5.01.030(6) applies the principal structure's, which is also what MCA 76-2-215(2)(f) requires, and the ADU footprint counts toward lot coverage. The county statute sets no state size cap, so this local cap governs, but 76-2-215(9)(a) defines an ADU as sitting beside a single-family dwelling "of greater square footage," which can bite where the flat 1,000 sq ft floor would exceed a small house.

Nothing in the Flathead County Zoning Regulations conditions an ADU on the owner living on the property: no owner-occupancy language appears in the ADU definition, the district use lists, the accessory-use standards, or the site plan review section. State law also forecloses it. MCA 76-2-215(2)(b) says a county may not "require that the single-family dwelling or the accessory dwelling unit be occupied by the owner," and (2)(c) bars requiring a familial, marital, or employment relationship between the occupants of the two units. Any contrary local rule in effect on or after 10-1-2025 would be void under 76-2-215(5). Ownership matters in one narrow, procedural place only: a short-term rental permit does not run with the land, so a new owner must reapply (FCZR 5.11.100).

FCZR 6.02.060 requires "1 space per 400 square feet gross floor area; maximum 2 spaces required" for an accessory dwelling unit, on top of the two spaces the principal one-family dwelling already needs. This is a live requirement, not a void legacy rule: Montana's county ADU statute deliberately omits the parking ban that the municipal statute MCA 76-2-345(2)(a) imposes on cities and towns, so counties may still require ADU parking. Flathead County left 6.02.060 untouched when Res. 955IR rewrote the ADU rules for the 2025 session in November 2025, and had separately trimmed other parking standards in May 2025 (Res. 955IP), which is affirmative evidence the county reads the statute the same way. If the ADU is also approved as a short-term rental, FCZR 5.11.090(2) requires two off-street spaces for it in addition to the principal dwelling's.

An ADU is a permitted use, never a conditional use, in every Flathead County zoning district that lists it. In fifteen districts (R-2, R-3, R-4, R-5, RA-1, RC-1, RR-1, BS, AL, HD, LBL, LL, LS, NF, WV) the listing carries the footnote "Administrative Site Plan Review required (See Section 2.10)": a 15-working-day staff review delegated to the Zoning Administrator under MCA 76-2-207 and 76-4-130, with no hearing and no neighbor notice, at a one-time $250 fee that matches the cap in MCA 76-2-215(4). In the agricultural and large-lot districts (AG-80, AG-40, AG-20, SAG-10, SAG-5, R-2.5, R-1) plus B-4, BM-1, BM-2, BR-4 and Rogers Lake, no county zoning permit is required at all, because Flathead County has no building department; building permits come from the Montana Department of Labor and Industry and sanitation review from DEQ or the Flathead City-County Health Department. One stale cross-reference survives: FCZR 5.01.020(3)(L) still reads "Dwelling unit, accessory (ADU) requires Administrative Conditional Use Permit" for R-2 through R-5 and RA-1, leftover 2015 text (Res. 955HR) that the amended district use lists, Section 2.10, and MCA 76-2-215(5) and (9)(b) all override.

An ADU is defined as a living unit occupied "with stays of 30 days or longer" (FCZR 8.05.160), so ordinary long-term renting is exactly what the use contemplates; the county caps neither rent nor tenancy length and imposes no primary-residence test. Renting an ADU for under 30 days is a different use: FCZR 5.11.020 provides that using "a dwelling, accessory dwelling, or a guest house as Short-term Rental Housing requires Administrative Conditional approval as outlined in Section 2.06.045," except in the zones where Chapter III lists short-term rentals as a permitted use. That administrative conditional process takes roughly six weeks, mails notice to owners within 150 feet, and gets bumped to the Board of Adjustment if a neighbor objects and the concern cannot be resolved; the permit does not run with the land, so a buyer must reapply (FCZR 5.11.100). Operators also need a State of Montana Public Accommodation License for a Tourist Home and must collect the state bed tax (FCZR 5.11.070, 5.11.080). Montana has no statewide short-term rental preemption and MCA 76-2-215(3) expressly preserves county regulation of it. The ADU cannot be sold separately: only one ADU is allowed per tract of record and it must be accessory to a principal single-family dwelling.