Montana · Administrative Rules of Montana Title 17
Montana DEQ reviews subdivisions and larger systems; county environmental health departments permit individual residential systems.
Minimum tank
One to three bedroom dwelling.
Design flow
Held two days for settling.
Cost index
Regional labour and permits.
Sets the design flow and the legal minimum.
Code sizes on bedrooms, not people.
Used to size the matching drainfield. Unsure? Sandy loam is the common default.
Mountain valleys with coarse gravel and cobble that drains very fast, glacial till on the plains, and shallow bedrock in the mountains. Nitrate loading to shallow gravel aquifers is the recurring regulatory concern.
Conventional trenches where soil is suitable. Where gravel is excessively coarse, sand-lined or engineered treatment is required to provide a treatment medium. Level 2 treatment is required in nitrate-sensitive areas.
| Dwelling | Design flow | Minimum tank | With disposal |
|---|---|---|---|
| 1 bedroom | 300 gpd | 1,000 gal | 1,250 gal |
| 2 bedrooms | 300 gpd | 1,000 gal | 1,250 gal |
| 3 bedrooms | 450 gpd | 1,000 gal | 1,250 gal |
| 4 bedrooms | 600 gpd | 1,250 gal | 1,500 gal |
| 5 bedrooms | 750 gpd | 1,500 gal | 1,750 gal |
Montana's subdivision review process is significant — creating new lots triggers DEQ review of wastewater capacity, and this is often the binding constraint on rural development rather than the individual system permit.
The state sets a floor. These Montana counties are where the largest septic populations sit, and where local requirements most often exceed it.
| County | What makes it different |
|---|---|
| Gallatin | Bozeman; rapid growth on coarse gravel with nitrate concerns |
| Flathead | Kalispell and lake area, strict shoreline rules |
| Missoula | Valley gravel over a shallow aquifer |
| Yellowstone | Billings area, mixed soils |
| Ravalli | Bitterroot valley development on gravel |
A three-bedroom home at 150 gallons per bedroom produces 450 gallons per day of design flow. On sandy loam that needs roughly 750 square feet of absorption area. On clay loam it rises to about 2,250 square feet, and on clay a conventional field is usually not permitted at all. Run your own soil class through the drainfield calculator.
Advanced treatment systems require operation and monitoring agreements. Use the pump schedule calculator to work out the interval for your tank size and household.
Regional labour and permit costs in Montana run about 100% of the national baseline. Here is what typical installations land at for a three-bedroom home:
| System type | Typical installed cost in Montana | When it applies |
|---|---|---|
| Conventional gravity | $9,000 – $16,000 | Good soil, adequate depth to water |
| Chamber system | $9,900 – $18,400 | Alternative to gravel trenches |
| Mound system | $17,100 – $33,600 | Shallow water table or thin soil |
| Aerobic treatment | $14,400 – $28,800 | Clay soil or small lots |
| Tank pump-out (service) | $320 – $650 | Every 3–5 years |
Replacing an existing system costs more than these figures — old-system removal and re-permitting add to every line. The replacement cost calculator covers that case, and the cost estimator adjusts all of the above for your bedroom count and soil.
Verify before you build. The figures above are state minimums compiled for planning, drawn from Administrative Rules of Montana Title 17, Chapter 36, Subdivisions and Onsite Subsurface Wastewater Treatment. County and health district rules routinely exceed them, code text changes, and only Montana DEQ — Subdivisions and Onsite Program or your local health department can confirm what applies to your parcel.
Send your sizing numbers to installers who work under Montana rules every day.