Oregon · Oregon Administrative Rules Chapter 340
Oregon DEQ administers the program, with several counties operating as contract agents delivering the program locally.
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.
Willamette valley clay with a high winter water table, coastal range and Cascade foothills with shallow rock and steep slopes, and volcanic soils east of the mountains that can be very fast-draining.
Standard trench systems where soil allows. Oregon makes heavy use of sand filters and pressure distribution on constrained sites, and ATT systems where nitrogen or separation limits apply.
| 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 |
Oregon requires a site evaluation before a permit, and the evaluation is valid for the specific parcel and design. Coastal and groundwater-limited areas face additional requirements, and DEQ has designated areas where nitrate reduction is required.
The state sets a floor. These Oregon counties are where the largest septic populations sit, and where local requirements most often exceed it.
| County | What makes it different |
|---|---|
| Clackamas | Portland fringe; contract county with local administration |
| Marion | Willamette valley clay and winter water table |
| Lane | Eugene area with varied soils |
| Deschutes | Bend; fast-draining volcanic soil over an aquifer, nitrate rules apply |
| Jackson | Southern Oregon, mixed conditions |
| Josephine | Rural development on rocky ground |
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.
ATT systems require ongoing maintenance and reporting to DEQ or the contract county. Use the pump schedule calculator to work out the interval for your tank size and household.
Regional labour and permit costs in Oregon run about 116% of the national baseline. Here is what typical installations land at for a three-bedroom home:
| System type | Typical installed cost in Oregon | When it applies |
|---|---|---|
| Conventional gravity | $10,400 – $18,600 | Good soil, adequate depth to water |
| Chamber system | $11,400 – $21,400 | Alternative to gravel trenches |
| Mound system | $19,800 – $39,100 | Shallow water table or thin soil |
| Aerobic treatment | $16,600 – $33,500 | Clay soil or small lots |
| Tank pump-out (service) | $370 – $750 | 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 Oregon Administrative Rules Chapter 340, Division 71, Onsite Wastewater Treatment Systems. County and health district rules routinely exceed them, code text changes, and only Oregon DEQ — Onsite Wastewater Management or your local health department can confirm what applies to your parcel.
Send your sizing numbers to installers who work under Oregon rules every day.