Building permits, zoning and septic rules in Lewis and Clark County, Montana

No. Lewis and Clark County has no residential building-permit program of its own for unincorporated land; the county's Community Development and Planning Department handles zoning, subdivision review and floodplain review instead of building codes¹. Montana's state permit exempts residential buildings with fewer than five dwelling units unless they serve transient guests, along with farm and ranch buildings and qualifying private garages and storage buildings². Covered projects must receive the state building permit before construction begins and before required plumbing, mechanical or electrical permits are issued². The state exemption applies outside certified local-government program areas; Helena is a certified local government limited to city limits, and the rest of the county falls under the State Building Codes Bureau¹¹. A homeowner who does their own plumbing in a personal-use, non-rental home needs no plumbing permit⁹. Montana's contractor-registration exemption for an owner working on their own property does not waive building, septic or other permits¹².

Since there is no county building department, your main point of contact for land-use questions is the Lewis and Clark County Community Development and Planning Department, which coordinates zoning, subdivision review, floodplain development, open lands and historic preservation¹³. Reach the department at 406-447-8374, or visit the City-County Building at 316 N. Park Ave., Room 230, Helena, MT 59623, Monday through Friday from 8:00 am to 5:00 pm¹³. Montana's Building Codes Program handles projects that require a state building permit, and the permit holder must notify the inspector when covered work is ready and provide access for required inspections². Ask the Planning Department about floodplain review and subdivision review before finalizing the site plan¹³.

Zoning is not countywide: Lewis and Clark County uses Part 1 and Part 2 zoning, and only parcels inside an established district are zoned^3,4. Part 2 regulations cover the Helena Valley and the Fort Harrison rural and urban growth areas, while the county has several Part 1 districts⁴. Use the county's interactive mapping site to identify the parcel's district before planning around setbacks or uses⁴. The county's zoning regulations do not set a minimum size or price for houses⁴. We could not confirm a countywide RV- or tiny-home occupancy rule, so ask the Planning Department what applies to the parcel¹³.

Septic systems in unincorporated Lewis and Clark County are permitted by the county's Environmental Services Division, part of Lewis and Clark Public Health, at their office in the Murray Building on Ninth Avenue in Helena⁶. Before you can get a septic permit you generally need a site evaluation of the soils, topography and water quality on your parcel, unless you already hold a DEQ subdivision approval or a previous site approval that still meets current regulations⁶. Site evaluation fees run $480 for a new site including the required non-degradation review, $400 for a replacement system or review of an existing system, and $80 for an individual non-degradation review alone, and the county has up to 20 working days to approve or deny the evaluation⁶. Once you have an approved site evaluation, permit fees depend on the system and installer: a certified installer pays $160 for a standard gravity system versus $320 for a non-certified installer, and only certified installers may install a pressure-dosed system at $320, an individual engineered system at $480, or a multi-user/public system at $720; a septic tank permit is $80 and a holding tank permit is $160 regardless of installer⁶. Start construction before your permit is issued and the fee doubles, with possible added penalties, and once issued a permit expires one year from the date it was granted⁶. The county permits individual septic tanks with drain fields, community treatment systems, holding tanks and outhouses⁵. Even if your subdivision already has a Montana DEQ Certificate of Subdivision Approval, that certificate does not replace the county's drainfield permit, so you still need one^6,14. For a private well, a new groundwater use of 35 gallons per minute or less and no more than 10 acre-feet a year can qualify for a permit exception instead of a water permit, while larger uses need a DNRC water permit¹⁰.

We could not confirm a countywide rule for living in an RV or tiny home on private land in unincorporated Lewis and Clark County. Zoning applies only inside designated Part 1 and Part 2 districts, so check the county's interactive map and ask the Community Development and Planning Department what applies to the parcel^3,4,13. A residential structure that needs wastewater disposal and is not connected to municipal sewer requires a county septic permit⁶. A land division that creates qualifying RV or mobile-home rental spaces can also trigger county subdivision review⁷.

1. Check the county's interactive map and ask Community Development and Planning what the parcel's zoning and floodplain status allow^4,13.
2. If the project divides land into qualifying parcels or creates multiple RV or mobile-home rental spaces, schedule the required subdivision pre-application meeting before applying⁷.
3. Order a septic site evaluation unless the parcel has a qualifying DEQ subdivision approval or prior site approval; the county may take up to 20 working days⁶.
4. Arrange a test hole, obtain a nearby well-water sample if required, and prepare a general facilities plan for the site evaluation⁶.
5. Confirm that the home fits the state exemption; covered projects need a state building permit before construction and before required trade permits².
6. If a state permit is required, notify the inspector when work is ready and provide access for required inspections².
7. Obtain an MDT approach permit or right-of-way agreement before building a new or modified driveway onto a state highway⁸.
8. If any part of the parcel may be in a flood zone, ask the local floodplain administrator whether a floodplain development permit is needed before fill, excavation or new construction¹⁵.
9. Obtain the septic permit before any construction, including well drilling or foundation excavation, to avoid a doubled fee and possible penalties⁶.

Ask Lewis and Clark Public Health's Environmental Services Division about a site evaluation before you commit to a parcel: only about 7.2% of the county's soils are rated free of severe limitations for a conventional septic drain field, and sites with shallow groundwater or bedrock face extra restrictions, since state and local rules require at least 4 feet of separation between a drain field trench and both the water table and bedrock^6,16. Check the county's interactive zoning map, since zoning in Lewis and Clark County only applies inside established Part 1 and Part 2 districts and rules differ sharply where it does apply⁴. About 51.1% of the county is federal land, which affects both access and what is actually available to buy¹⁶. Only 26.8% of the county's land sits at 15% slope or gentler, so ask about slope and access before you assume a building site will be easy to grade¹⁶. Confirm how your access road connects: a new or modified driveway onto a state highway needs a Montana Department of Transportation approach permit or right-of-way agreement, while a driveway onto a county road is a local matter to confirm with the county⁸. Lewis and Clark County's median property tax bill is about $2,886 a year at an effective rate near 0.8%, and typical homes in the county are valued around $471,902¹⁶.