Bolthole Index

Gallatin County, Montana

Building permits, zoning and septic rules in Three Forks, Montana

Three Forks, Montana has city zoning, and commercial construction requires a state building permit unless a state exemption applies.12 City Hall is the starting point for zoning and floodplain applications; City operators handle water and sewer connections.34 For onsite wastewater in Three Forks, Gallatin County, septic permits come from the Gallatin City-County Health Department.56

At a glance

Building permits
Commercial construction: state building permit, subject to exemptions; ask the state program about your home project278
Zoning
City zoning applies inside Three Forks; start at City Hall13
Septic permits
Required for onsite systems; Gallatin City-County Health Department56
Water and sewer
City connections require City operators; new construction connections carry impact fees4
Manufactured homes
Conditional: at least 900 square feet, permanent foundation and other city criteria9
Accessory dwellings
State law requires a by-right ADU on a single-family lot in a municipality exercising zoning powers, subject to applicable standards10
RV and tiny-home living
We could not confirm residential occupancy rules; ask City Hall before buying for that use3

Who to call

Zoning

Three Forks City Hall

City zoning applications and building, utility and floodplain information

406-285-3431

206 Main Street

Building permits

Montana Building Codes Program

State building-permit applicability, plan review and inspections

(406) 841-2056

[email protected]

2705 Skyway Drive

Also useful

Three Forks Public Works

City water and sewer connections; coordinate new construction through City Hall

(406) 285-3408

306 Railway Avenue

Floodplain permits

Three Forks Floodplain Administrator

City floodplain questions, maps, elevation certificates and floodplain applications

406-285-3431

Septic permits

Gallatin City-County Health Department (Environmental Health)

Wastewater-system permits, registration, plan review, and inspections in Gallatin County

406.582.3120

[email protected]

400 E. Babcock Street

Contact details are copied from each office’s official page and were checked against it on October 12, 2026.

Off-grid gear

Going off-grid in Three Forks means bringing your own water and power. The kit we’d start with:

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Does Three Forks have zoning, and what does it mean for your lot?

Three Forks publishes city zoning regulations and a zoning-map chapter; City Hall handles zoning information and applications.1113 Ask City Hall to identify the parcel's district and confirm lot size, setbacks, height and lot coverage before settling on a house plan.3

Three Forks has specific mobile/manufactured-home placement criteria: at least 900 square feet of occupied space, a permanent foundation, ground anchors, removed running gear, City-compliant utilities and residential-style siding and roofing.9 The home must bear a HUD seal, be in good repair and have been manufactured no more than 10 years before the building-permit application.9 Those mobile-home standards do not establish a minimum size for every site-built house; ask City Hall for the standards applicable to your proposed dwelling.93

Do you need a building permit in Three Forks, and who issues it?

For commercial construction in Three Forks, the City directs applicants to obtain a state building permit in addition to zoning approval and any applicable floodplain permit; state exemptions may apply.2 Start with Three Forks City Hall for local approvals and the Montana Building Codes Program for state building-permit applicability.38

Within state building-code jurisdiction, Montana exempts residential buildings containing fewer than five dwelling units unless they serve transient guests.7 A state building-permit exemption does not answer the City zoning or utility-connection questions, and separate electrical, plumbing or mechanical permits may be required.1213 For a required state building permit, submit construction plans and the Building Permit/Plan Review Application; the permit follows satisfactory responses to review comments and payment of fees.13 The permit holder must notify the inspector when work is ready and provide access for required inspections.14

How do septic permits work inside Three Forks?

For a Three Forks property using onsite wastewater, a drainfield permit is required from the local health department even if the property already has a Certificate of Subdivision Approval, or COSA.5 The Gallatin City-County Health Department administers local wastewater permitting; City zoning approval does not replace that septic permit.65

Gallatin County requires a septic permit before installation, a site evaluation by a registered site evaluator or Montana-registered professional engineer, and a registered installer on-site during installation.15 The application includes the wastewater treatment system form, property information request form, fee payment and scaled site plan.15

First establish whether the proposed home will connect to City sewer or use onsite treatment: Three Forks provides sewer service, and only City operators may connect to the mains.16 A new sewer connection and a new septic installation therefore call for different offices and approvals.165

Can you live in an RV or tiny home in Three Forks?

We could not confirm permission for residential RV occupancy, temporary camping on private land or tiny-home occupancy inside Three Forks; ask City Hall about the exact proposed use before buying a parcel or moving a unit onto it.3 Three Forks' mobile/manufactured-home criteria provide an answer for that housing category, including the 900-square-foot minimum, but do not establish permission for an RV residence.9

An accessory dwelling has a separate statewide framework: a Montana municipality exercising zoning powers must allow at least one ADU by right on a lot containing a single-family dwelling, subject to applicable building, fire and public-health standards.10 A detached or attached ADU may not exceed 75% of the primary home's gross floor area or 1,000 square feet, whichever is less.10 The municipality may require water and sewer will-serve letters; ask City Hall how the proposed ADU meets the site and service requirements.103

What are the steps to building a home in Three Forks?

  1. Bring the parcel and proposed use to City Hall for zoning and floodplain information, and ask which City applications apply.3
  2. Call City Hall about new construction water or sewer connections; the City says zoning approval and possibly floodplain approval typically come first.12
  3. If onsite wastewater is proposed, arrange the site evaluation and obtain the Gallatin City-County Health Department septic permit before installation.15
  4. Confirm state building-permit applicability with the Montana Building Codes Program and identify separate trade permits before scheduling work.813
  5. Budget for connection charges: new construction connecting to City water or sewer carries impact fees payable when the zoning permit is issued, and only City operators may connect to the mains.4
  6. For a project requiring state inspections, notify the inspector when work is ready and provide access.14

What should you check before buying land in Three Forks?

Ask the Three Forks Floodplain Administrator to check the parcel before planning grading, fill or construction; Montana requires local floodplain approval for covered development in a designated Special Flood Hazard Area.1718 City Hall holds elevation certificates for some Three Forks properties, so request any existing certificate for the address.17

If a private well is proposed, distinguish City service approval from Montana water-right requirements: a groundwater development using no more than 35 gallons per minute and 10 acre-feet annually may qualify for a permit exception, but qualification is not automatic.19 Beginning in 2026, the exempt-well route requires a Notice of Intent to DNRC before water use, with a $400 filing fee.19

For a parcel outside city limits, use the Gallatin County guide for county land-use requirements rather than carrying Three Forks zoning across the boundary.20 Ask City Hall separately about access to City streets; approaches onto state highways require MDT approval for new or modified access.321

Watch-outs

  • A state residential building-permit exemption does not replace Three Forks zoning approval or applicable utility and septic approvals.7125
  • We could not confirm Three Forks' residential RV or tiny-home occupancy rules; ask City Hall before relying on either as housing while building.3
  • Ask City Hall how an ADU connection is classified: Montana's municipal ADU statute prohibits ADU impact fees, while the City describes impact fees for new construction connections generally.104

Sources

  1. Three Forks, MT Laws (Three Forks), quoted and re-checked
  2. threeforksmontana.us (Three Forks), quoted and re-checked
  3. Three Forks City Hall (threeforksmontana.us)
  4. threeforksmontana.us (Three Forks), quoted and re-checked
  5. deq.mt.gov (Three Forks), quoted and re-checked
  6. Wastewater (Septic) Treatment - Healthy Gallatin, quoted and re-checked
  7. Building Permits (Montana statewide rule), quoted and re-checked
  8. Montana Building Codes Program (bsd.dli.mt.gov)
  9. 11-6A-11: Specific Requirements for Mobile Homes (Three Forks), quoted and re-checked
  10. 76-2-345. Accessory dwelling units -- regulations -- restrictions, MCA (Montana statewide rule), quoted and re-checked
  11. Three Forks, MT Laws (Three Forks), quoted and re-checked
  12. threeforksmontana.us (Three Forks), quoted and re-checked
  13. Building Permits (Montana statewide rule), quoted and re-checked
  14. Building Permits (Three Forks), quoted and re-checked
  15. Septic (Wastewater Treatment), quoted and re-checked
  16. Three Forks Public Works (threeforksmontana.us)
  17. Three Forks Floodplain Administrator (threeforksmontana.us)
  18. Stream Permitting (Montana statewide rule), quoted and re-checked
  19. Exempt Well Updates and Information (Montana statewide rule), quoted and re-checked
  20. Frequently Asked Questions | Gallatin County, MT, quoted and re-checked
  21. Approach Manual for Landowners and Developers (Montana statewide rule), quoted and re-checked

Written October 12, 2026 from the rules quoted below, each taken from an official source and re-checked by an independent reviewer. This is research, not legal advice: rules change, so confirm with the office before you buy or build.

Answers from the rulebook for Three Forks

Three Forks’s own rules

  • Local zoningPresent

    Land inside Three Forks city limits; the City publishes Title 11, Zoning Regulations.

    “TITLE 9 WATER AND SEWER [...] TITLE 10 BUILDING REGULATIONS [...] TITLE 11 ZONING REGULATIONS [...] TITLE 12 FLOOD CONTROL”
  • Zoning districtsPublished

    Land inside Three Forks city limits; the City code publishes Title 11 Zoning Regulations and a zoning-map chapter.

    “CHAPTER 1 TITLE; JURISDICTION; ZONING MAP”
  • Manufactured homesLimited

    Placement of a mobile/manufactured home on a residential lot in Three Forks city.

    • A zoning official must review the criteria before issuance of a zoning permit.
    • The home must have at least 900 square feet of occupied space.
    • It must be a double-section or appropriately designed single-section unit, have City-compliant utilities, siding and roofing customary for site-built residences, a permanent foundation, ground anchors, and removed wheels, axles and hitch mechanisms.
    • It must have been manufactured no more than 10 years before the building-permit application, bear a HUD seal, and be in good repair.

    minimum occupied space: >= 900 square feet · maximum age at building-permit application: <= 10 years

    “Mobile homes shall have a minimum of nine hundred (900) square feet of occupied space”
  • Mandatory utility hookupConditional

    New construction inside Three Forks city limits seeking a new connection to the City's water and/or sewer mains.

    • A zoning permit and possibly a floodplain permit are typically obtained first.
    • Only City operators may connect to the City's mains.
    • New connections are charged impact fees; fees are payable when the zoning permit is issued.
    • The connection form requires at least 48 hours' notice before physical connection approval.

    new connection application fee: 250 USD (The page also states that incidental parts and asphalt replacement may be charged separately and may vary.)

    “Only City-operators may connect to the City's mains.”
  • Permit processPartial

    New construction and new water/sewer connections inside Three Forks city limits.

    • Call City Hall for new construction connections.
    • Typically obtain a zoning permit and possibly a floodplain permit first.
    • Only City operators may connect to City mains.
    • Impact fees are payable when a zoning permit is issued.
    “If you have a new construction to be connected, please call 406-285-3431. Typically you have already obtained a Zoning Permit, possibly a Floodplain Permit, and know this. However, you may have an existing structure to connect, or an existing connection to be replaced. Only City-operators may connect to the City's mains.”

Rules another office applies inside Three Forks

Three Forks does not set these itself; the county, a health district or the state does, and they apply to every address inside its limits.

  • Building permitRequired

    Commercial construction inside Three Forks city limits.

    • The City's page says commercial construction must obtain a State Building Permit in addition to zoning and possibly flood permits.
    • State exemptions may apply.
    “you must also obtain a State Building Permit.”
  • InspectionsRequired

    Projects inside Three Forks city limits for which a state building permit and required inspections apply.

    • The permit holder must notify the inspector when work is ready and provide access for required inspections.
    “provide access for required inspections.”
  • Septic permitRequired

    Installation of a drainfield/septic system inside Three Forks city limits where onsite wastewater is used.

    • A COSA does not replace the drainfield permit.
    • The permit is issued by the local health department.
    “a drainfield permit is still required by the local health department.”
  • Subdividing landRegulated (with conditions)

    Land divisions and qualifying developments inside Three Forks city limits that fall within Montana subdivision-sanitation review.

    • DEQ reviews divisions creating parcels under 20 acres.
    • DEQ also reviews condominiums and recreational camping vehicle and mobile-home parks regardless of parcel size.
    • A COSA addresses water, wastewater, solid waste and stormwater; local health departments still require drainfield permits.

    parcel threshold: < 20 acres (RV and mobile-home parks are reviewed regardless of parcel size.)

    “reviews divisions of land comprising less than 20 acres”

Still unanswered

We looked and could not find an official answer yet. Silence is not permission; ask Three Forks or Gallatin County directly.

  • Owner-builder exemption — Owner-builder exemptions for construction inside Three Forks city limits.
  • Certificate of occupancy — Certificate-of-occupancy requirements for buildings inside Three Forks city limits.
  • Minimum dwelling size — Single-family dwelling dimensional standards inside Three Forks city limits.
  • Minimum lot size — Single-family residential lots inside Three Forks city limits.
  • Setbacks — Residential setbacks inside Three Forks city limits.
13 more unanswered
  • Lot coverage
  • Building height
  • Living in an RV
  • Camping on your land
  • Tiny homes
  • Accessory dwellings
  • Septic inspection
  • Off-grid sanitation
  • Well permit
  • Agricultural exemption
  • Driveway access
  • Building in a floodplain
  • Short-term rentals

Each answer was extracted from an official source and independently re-checked against that source by a second reviewer. That is an evidence check, not legal advice. Rules change; confirm with the authority before you buy or build.

Unanswered questions are usually a sign that Three Forks publishes no rule on the point, not that none exists. Gallatin County’s page lists the county offices to call.

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