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Yellowstone County, MT Accessory Structures: ADU Rules (2026)

Some Restrictions

Compiled from the official code textEditor Martyn O'NeillLast verified September 2026

ADU rules in Yellowstone County, MT, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.

Key Facts

Guest house allowed: Within an accessory building
Kitchen: No kitchen facilities allowed
Separate rental: Not permitted as dwelling
Rural unzoned land: No county ADU rule
Contact: City-County Planning

ADU Eligibility Quick-Check

An Accessory Dwelling Unit (ADU) is a smaller, independent living unit on the same lot as a single-family home: converted garages, detached cottages, basement or attic conversions, and attached additions all qualify. Most states have streamlined ADU permitting since 2020, but local rules still control size, height, and setback.

Is your lot zoned single-family or multi-family residential?
Does your lot have at least 1,200 sq ft of buildable space?
Is your existing home owner-occupied (in most states this is not required, but some require it for ADU permits)?
Will the ADU stay under your state's size cap (commonly 850-1,200 sq ft for detached)?
Can you provide one off-street parking space (waived in many states within 0.5 mi of transit)?

If you answered 'yes' to most of these, you likely qualify in Montana under recent ADU streamlining laws. The Yellowstone County, MT rules below cover local size, height, and setback requirements.

Summary

In Yellowstone County's zoning area around Billings, "accessory living quarters" (guest houses) are allowed within an accessory building but may have no kitchen and may not be rented as a separate dwelling. A full second dwelling depends on the zoning district. Rural unzoned land sets no such rule.

City-specific rules exist: Billings has its own adu rules rules that differ from Yellowstone County's county-level regulations. If you live in Billings, check the city-specific page instead.

"Accessory living quarters" means living quarters within an accessory building for the sole use of the family or of persons employed on the premises... Such quarters have no kitchen facilities and are not rented, leased, or otherwise used as a separate dwelling unit.

View official code

Official source re-checked September 7, 2026: the cited page had not changed since it was quoted.

Full Breakdown

Yellowstone County exercises zoning jurisdiction in the district around Billings and, through the adopted Zoning Ordinance (Title 17), defines "accessory living quarters" as quarters within an accessory building for family, employees or temporary guests only. The regulation states such quarters "have no kitchen facilities and are not rented, leased, or otherwise used as a separate dwelling unit" and includes the "guest house." A true rentable second dwelling (an ADU with a kitchen) is not authorized under this definition and would require a permitted multifamily use in the district. On unzoned rural county land, the county sets no ADU rule and defers to state building code and septic (DEQ) approval. Confirm your parcel's district with County/City-County Planning.

Violations & Fines

Building or renting a non-conforming accessory dwelling in the zoning area is a zoning violation handled by County Code Enforcement (406-652-2050); remedies include permit denial and orders to remove unpermitted living quarters.

Frequently Asked Questions

Can I add a rentable ADU with a kitchen in the county zoning area?

No. The county's "accessory living quarters" definition bars kitchen facilities and separate rental. A true rentable dwelling requires a district that permits multifamily use, not an accessory building.

What about ADUs on rural, unzoned county land?

The county sets no zoning rule there, but you still need state building-code compliance and DEQ/RiverStone Health septic (subsurface wastewater) approval before adding a dwelling.

Sources & Official References

Other rules in Yellowstone County

Shed & Carport RulesSome Restrictions Tiny HomesHeavy Restrictions ADU Owner OccupancySignificant Restrictions Garage ConversionsSome Restrictions Noise Ordinances: Amplified Music & Outdoor SoundSome Restrictions Noise Ordinances: Decibel LimitsSome Restrictions Parking Rules: EV ChargingSome Restrictions Parking Rules: Snow Removal Parking RulesSome Restrictions Fence Regulations: Fence Permit RequirementsSome Restrictions Fence Regulations: Retaining WallsLight Restrictions

All Yellowstone County rules

How Yellowstone County compares: Easiest Cities to Build an ADU·Compare Yellowstone County to another location·View the Montana accessory structures overview

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ADU Rules in Cities Across Yellowstone County

Billings, MT