Building permits, zoning and land rules in Petroleum County, Montana

Petroleum County does not impose a building permit beyond Montana's requirements, and the county has not adopted its own building code^1,2. Montana exempts farm and ranch buildings and residential buildings with fewer than five dwelling units unless they serve transient guests³. A typical private single-family home therefore needs no county or state building permit³. Montana's uniform code still governs projects within state jurisdiction, including nonexempt projects in Petroleum County⁸. For a state-permitted project, obtain the building permit before construction and before related plumbing, mechanical or electrical work begins³.

Petroleum County has no county-adopted building code and no full-time building official, so there is no county office that issues building permits². If a project is not exempt, the state issues the building permit: contact the Montana Building Codes Program at 406-841-2056 or buildingcodes@mt.gov³. State applicants submit plans and the Building Permit/Plan Review Application; plan review averages three weeks, and written responses to review comments and payment are required before issuance³. State fees depend on project valuation, with an additional plan-review fee equal to 35 percent of the building-permit fee⁹.

What stays with the county: the Road Department for county-road access, and the Board of Commissioners for subdivision and Growth Policy questions^10,11. The county's Planning Department also runs floodplain permits, which cost $250 per permit under the 2026 fee schedule¹². Septic and drainfield permits are separate again, with Montana DEQ and the local health department⁴.

Petroleum County has no countywide zoning, including in unincorporated areas¹. Because there is no zoning ordinance, no county rule sets minimum dwelling size, setbacks, lot coverage or building height on an unincorporated parcel¹. Zoning being absent does not remove subdivision review, sanitation approval, floodplain controls or other non-zoning requirements¹. New lots remain subject to the county's subdivision regulations and review against its Growth Policy¹.

For a subdivision, Montana's sanitation rules require each proposed lot to be large enough for the septic system, a drainfield replacement area, the water supply and all permanent structures, and the reviewing authority may require larger lots to protect health or water quality¹³. Rules inside the Town of Winnett are separate from the countywide answer; we could not confirm them, so ask the town before buying a lot within its limits.

Montana DEQ handles subdivision sanitation approval in Petroleum County⁴. For property subject to that review, DEQ issues the Certificate of Subdivision Approval covering the approved use, water source, wastewater method and stormwater mitigation⁴. A COSA must be issued before facilities requiring sewer or water are developed, but that rule does not require every parcel to connect to municipal utilities⁴. A separate drainfield permit remains required from the applicable local health department, which we could not identify; ask DEQ or the county commissioners who currently serves Petroleum County⁴. About 21.6% of county soils lack severe limitations for a conventional drainfield, so obtain a site evaluation rather than assuming a standard system will work¹⁴.

Unincorporated Petroleum County has no county rule on long-term RV living, tiny homes, manufactured homes or accessory dwellings, because the county has no zoning ordinance¹. That removes the county as a barrier, not the other requirements: any dwelling that generates wastewater still needs Montana DEQ sanitation approval and a drainfield permit from the local health department⁴. Montana's camping limits for state public-use sites apply to state recreation land, not private property, so they do not answer residential RV use here¹⁵.

Private covenants, a lender or an insurer may still set limits, and the Town of Winnett's rules for a lot inside the town are unconfirmed. Before buying for one of these uses, ask the Board of Commissioners to confirm nothing parcel-specific applies¹¹.

1. Confirm whether the state exemption applies; residential buildings with fewer than five dwelling units are exempt unless they serve transient guests³.
2. If a state permit is required, submit plans and the Building Permit/Plan Review Application before construction or related trade work begins³.
3. Resolve DEQ sanitation approval before developing facilities that require water or sewer, then obtain the separate drainfield permit from the applicable local health department⁴.
4. For a qualifying well pumping no more than 35 gallons per minute and 10 acre-feet yearly, file the required groundwater notice before use⁷.
5. If the parcel touches a stream, ask the county floodplain administrator whether it lies in a designated Special Flood Hazard Area and, if so, get a floodplain permit before building⁵.
6. If creating a lot, start subdivision review with the Board of Commissioners^1,11.
7. On state-permitted work, call the inspector at least 24 hours before the requested inspection; the inspector has 72 hours to perform it¹⁶.

Petroleum County has about 554 residents, so verify contractor availability and travel to services before buying¹⁴. About 37.4% of county land is federally owned, making parcel boundaries and legal access worth checking carefully¹⁴. Only 21.6% of county soils lack severe limitations for a conventional drainfield, so make sanitation feasibility part of due diligence¹⁴.

For water, a well using 35 gallons per minute or less and no more than 10 acre-feet per year may qualify for the exempt-well exception, which since January 1, 2026 requires a Notice of Intent filed with DNRC and a $400 form fee¹⁷. A new or modified driveway onto a state highway needs an approach permit or right-of-way agreement from the Montana Department of Transportation; the county-road driveway process could not be confirmed¹⁸. Development in a designated Special Flood Hazard Area needs a floodplain permit, and the floodplain administrator decides whether the stream has one⁵.

The median property-tax bill is about $1,261 yearly, with an effective rate near 0.7%¹⁴. Severe drought occurs in roughly 12.1% of years, a practical concern for a private water plan¹⁴. The nearest city of 100,000 or more is about 96 miles from the county center¹⁴.