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Billings, MT Accessory Structures: ADU Rules (2026)

Some Restrictions

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

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

Key Facts

Units allowed: 1 per residential lot
By-right zones: N1, N2 (special review in N3)
Min. lot size: 3,000 sq ft
Max detached size: 80% of primary or 750 sq ft
Occupancy rule: Owner in primary or ADU 6+ months/yr

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 Billings, MT rules below cover local size, height, and setback requirements.

Summary

Billings allows one accessory dwelling unit per residential lot by right in N1/N2 zones, by special review in N3, with owner-occupancy and size caps.

G.Accessory dwelling units.1.Applicability.(a)Accessory dwelling units (ADUs) are permitted by right in the N1 and N2 zone districts and are allowed by special review in the N3 zone district.(b)Accessory dwelling units in applicable zones are permitted on any parcel where a single dwelling unit is permitted or currently exists.(c)The minimum lot size for a lot that has both a primary dwelling unit and an accessory dwelling unit is three thousand (3,000) square feet.

View official code

Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: Code of Ordinances: Supplement 66 Update 2).

Full Breakdown

Billings Unified Development Code Sec. 27-1008(G) permits one ADU per residential lot on a parcel of at least 3,000 square feet. The property owner must occupy the primary or accessory unit as their principal residence at least six months a year and record a deed restriction with the Yellowstone County Recorder. A detached ADU is capped at 80 percent of the primary dwelling's gross floor area or 750 square feet, whichever is less, and cannot exceed the primary structure's height or footprint. One dedicated off-street parking space is required in addition to what the primary home needs.

Violations & Fines

Building an ADU without planning approval or exceeding the size/height caps is a zoning violation subject to the city's standard code enforcement and permit-revocation process under the Unified Development Code.

Frequently Asked Questions

Can I rent out both units in a Billings ADU property?

No. The property owner must live in either the primary or accessory unit as a principal residence at least six months a year; the owner-occupied unit cannot be rented to a third party.

How big can a detached ADU be in Billings?

No more than 80 percent of the primary dwelling's gross floor area, or 750 square feet, whichever is smaller, and it can't exceed the main house's height or footprint.

Sources & Official References

[1]Billings Unified Development Code, Sec. 27-1008

Other rules in Billings

Parking Rules: Overnight ParkingSignificant Restrictions Noise Ordinances: Quiet HoursSome Restrictions Shed & Carport RulesSome Restrictions ADU Owner OccupancySome Restrictions Noise Ordinances: Amplified Music & Outdoor SoundSome Restrictions Noise Ordinances: Barking DogsSome Restrictions Parking Rules: EV ChargingSome Restrictions Parking Rules: Driveway RulesSome Restrictions

All Billings rules

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

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