Montana Septic Permits & DEQ Rules, Explained

Short answer: In Montana, your county (usually environmental health) issues the permit for a home septic system. The Montana Department of Environmental Quality (DEQ) sets statewide design standards and licenses septic pumpers. A routine pump-out of an existing tank usually does not need a new permit. A new system, a replacement tank or drain field, or a major change almost always does. A DEQ subdivision approval (a COSA) does not replace the county permit.

We’re a directory, not a regulator. Confirm every step with the county where the property sits—rules differ, and they change.

Who does what

• County or city-county environmental health issues most individual home septic permits, reviews or does site evaluations, and inspects installations. Call this office first.
• Montana DEQ writes the statewide design standards in Circular DEQ-4 (2023 edition) and licenses septic pumpers.
• DEQ’s Subdivision Program reviews many divisions of land under 20 acres, plus condominiums and RV or mobile-home parks of any parcel size, for sanitation (water, sewage, solid waste, and storm drainage). See Engineering Infrastructure & Subdivisions. Lots 20 acres and larger often go through a local review instead.
• Installers and site evaluators: many counties require a registered or licensed installer—and sometimes a registered site evaluator or a Montana professional engineer—to be on site. Rules differ. There isn’t one statewide installer license that covers every county.

When you typically need a county permit

Confirm locally. Across the official pages we checked, a permit is typically required for:

• Installing a new system
• Replacing a tank or drain field
• Expanding, relocating, or otherwise altering a system
• Adding bedrooms or living units that increase wastewater flow (some counties are explicit that adding a bedroom, kitchen, or bathroom is an alteration)
• Some repairs to major components (this is county-dependent)

Two local examples, not statewide rules: in Lewis and Clark County, you must have the septic permit before any construction on the parcel, including drilling a well or digging a foundation. In Yellowstone County, both new and repaired systems must be inspected and permitted before the system is placed into use.

When you typically don’t

Hiring a DEQ-licensed pumper for a routine pump-out of a system that isn’t being altered usually does not need a new permit. In some counties, flushing a plugged line or repairing a broken line also doesn’t; work on major components, such as replacing or expanding drain-field lines, does.

If you’re unsure, call the county.

What a COSA is

A Certificate of Subdivision Approval (COSA) is DEQ sanitation approval for a lot. It describes the allowed use (for example, residential), the water source, where wastewater goes, and stormwater requirements. Lots created after 1961 and under 20 acres often have a COSA—or they may have sanitary restrictions instead. Your local health department is the best place to check and to get a copy of the COSA and lot layout.

Even if you have a COSA, you still need a local drain-field / septic permit. DEQ’s own FAQ is explicit about that on the subdivisions page. Don’t move a drain field or add a second living unit without asking the county—and, if the COSA language would change, DEQ.

A typical path (this varies by county)

1. Confirm which county the property sits in. Permits follow the property, not the mailing address.
2. Ask the county whether the parcel already has a COSA, a prior permit, or needs a site evaluation. Some counties start with a records request (for example, a Land Research Request in Flathead, or a property information request with the application in Gallatin).
3. If there is no prior approval, a site evaluation is usually required (a soil test pit, sometimes a percolation test). Who digs and who must be present varies.
4. Groundwater monitoring may be required if seasonal high water is a concern. Monitoring windows are seasonal and county-specific—don’t treat one county’s dates as a statewide rule.
5. Apply for the county permit and wait for approval before any construction.
6. Use a county-approved installer if the county requires one.
7. Schedule the county inspection before using the system.

For general system care, see DEQ’s homeowner guide to septic systems. It is not a permit manual.

Find your county

Start with our county page for local context, then open the official office for applications and current fees. We don’t publish fee amounts here so they don’t go stale.

• Missoula County — Our page · Missoula Public Health — Septic & Well Permits. A permit is often needed before other permits. Groundwater monitoring may be required; their page cites 4 feet of separation to groundwater.
• Flathead County — Our page · Sewage & Septic and the permit application. Start with a Land Research Request. County regulations were updated April 1, 2026.
• Gallatin County — Our page · Healthy Gallatin — Septic Systems. A permit is required before installation. Site evaluations are done by a Gallatin-registered site evaluator or a Montana PE, and a Gallatin-registered installer must be on site. Public systems (as DEQ defines them on that page) are primarily a DEQ responsibility.
• Ravalli County — Our page · Septic Permits. All owners of record must sign. If there is no DEQ approval, the owner schedules a site evaluation and hires an excavator for a soil test hole with the county present.
• Lake County — Our page · Lake County Wastewater Treatment. Use the official page for forms and current requirements.
• Lewis and Clark County — Our page · Septic System Programs. The permit is required before any construction, including the well or foundation. Permits expire one year after issuance, per their page.
• Yellowstone County — Our page · RiverStone Health — Septic Permits + Subdivision Review. RiverStone is the city-county health department. Confirm prior DEQ or local approval, get a Permit Tracking Sheet at the courthouse, then apply. Repairs use a repair application. Systems are inspected before use. Parcels under 20 acres typically go through DEQ review; 20 acres or more typically go through local review.
• Cascade County — Our page · Cascade County septic systems.
• Park County — Our page · Park County Environmental Health. Start on this department page for forms and current requirements.
• Silver Bow County — Our page · Butte-Silver Bow septic permits.

Elsewhere in Montana, start with the local environmental health office. One official starting point is Lincoln County septic permitting. More statewide references: DEQ water resources / circulars and the current licensed pumpers list (DEQ list dated May 20, 2026).

County pages can change. If a link is out of date, use the current official page rather than guessing from this guide.

FAQ

1. Can Montana Septic Pros pull a permit for me? No. We are a directory. We can help you find a licensed pumper; the county issues the permit.
2. Do I still need a county permit if I have a COSA? Yes. DEQ says a local drain-field permit is still required.
3. Does a routine pumping job need a permit? Usually no, if you aren’t altering the system. Confirm with the county.
4. Do pumpers need a DEQ license? Yes. Browse our licensed pumper list and confirm current status with DEQ if needed. Licenses expire December 31 each year.
5. Are permit fees the same statewide? No. Counties set many local fees, and they change. Check the official county page—we don’t publish amounts here so they don’t go stale.
6. Who should I call first? The environmental health office in the county where the property sits. Then a licensed local pumper, or a registered installer or evaluator if the county requires one.

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