Oregon · OAR 340-071-0220
Septic system requirements in Oregon
Tank sizes, drainfield rules, setbacks, permits, and real costs — pulled from Oregon's own code rather than a national average.
- 3-bedroom tank
- 1,000 gal
- Design flow
- 150 gpd/BR
- Typical install
- $12,000–$32,000
- Permit fee
- $500–$2,000
- Well setback
- 100 ft
- Permitted by
- State + local
Septic tank sizes required in Oregon
Oregon sizes systems on a design flow of 150 gallons per day per bedroom. The tank must meet both the bedroom table below and a 2× daily-flow detention requirement — whichever produces the larger number governs.
| Bedrooms | Design flow | Code minimum | Recommended |
|---|---|---|---|
| 1 bedroom | 150 gpd | 1,000 gal | 1,000 gal |
| 2 bedrooms | 300 gpd | 1,000 gal | 1,000 gal |
| 3 bedrooms | 450 gpd | 1,000 gal | 1,000 gal |
| 4 bedrooms | 600 gpd | 1,000 gal | 1,250 gal |
| 5 bedrooms | 750 gpd | 1,250 gal | 1,500 gal |
| 6 bedrooms | 900 gpd | 1,500 gal | 2,000 gal |
Source: OAR 340-071-0220. The recommended column rounds the governing requirement up to a tank size manufacturers actually stock.Run your own numbers →
Drainfield size and cost by soil type
For a three-bedroom home in Oregon at 450 gpd. This single table explains most of the variation between quotes — the area scales inversely with how fast your soil accepts water, and the drainfield is the largest line item in the job.
| Soil | Loading rate | Absorption area | With reserve | Typical total install |
|---|---|---|---|---|
| Medium sand | 0.8 gpd/ft² | 563 ft² | 1,126 ft² | $12,800 |
| Sandy loam | 0.5 gpd/ft² | 900 ft² | 1,800 ft² | $16,000 |
| Loam | 0.4 gpd/ft² | 1,125 ft² | 2,250 ft² | $18,100 |
| Silt loam | 0.3 gpd/ft² | 1,500 ft² | 3,000 ft² | $21,600 |
| Clay loam | 0.2 gpd/ft² | 2,250 ft² | 4,500 ft² | $28,700 |
Estimates use Oregon's cost index of 1.05× the national average. Build a line-item estimate →
Permits and who issues them
Oregon DEQ permits directly in some counties and contracts the programme to others. Which applies depends on your county.
| Permit required | Yes |
|---|---|
| Typical fee | $500 – $2,000 |
| Soil evaluation | Required before permit |
| Licensed installer | Required |
| Turnaround | 4–10 weeks |
| Ongoing inspection | Annual reporting required for ATUs and sand filters |
| Reserve area | 100% replacement area required |
Minimum setback distances
Measured horizontally from the drainfield unless noted. Local rules are frequently stricter, and on a small lot these numbers — not the soil — often decide the layout.
| Private water well | 100 ft |
|---|---|
| Well (septic tank) | 50 ft |
| Property line | 10 ft |
| Surface water | 100 ft |
| Building foundation | 10 ft |
Regulatory contact
Oregon DEQ — Onsite Wastewater Treatment Systems
www.oregon.gov · 503-229-5696
What makes septic hard in Oregon
Willamette Valley clay and a winter water table within a foot of the surface make gravity trenches marginal on a lot of otherwise good land.
Coastal dune sand has the opposite issue — too fast, with little treatment.
Land-use rules mean a septic approval and a building approval are separate hurdles, and failing either blocks the other.
Things that catch people out
- An ADU is treated as adding real flow: the minimum design flow for a house plus an ADU is 750 gpd, which often means a new drainfield.
- Oregon permit fees are among the highest in the country — $500 to $2,000 is normal before a shovel moves.
Oregon septic questions
What size septic tank do I need in Oregon?
A three-bedroom home in Oregon requires a minimum 1,000 gallon tank under OAR 340-071-0220. Design flow is 150 gallons per day per bedroom, and the tank must also hold 2 times daily flow, so the practical recommendation is 1,000 gallons.
Do I need a permit for a septic system in Oregon?
Yes. Oregon DEQ permits directly in some counties and contracts the programme to others. Which applies depends on your county. Fees typically run $500 to $2,000, with a turnaround of about 4–10 weeks. A soil evaluation or percolation test is required before a permit is issued.
How much does a septic system cost in Oregon?
A conventional system for a three-bedroom home typically runs $12,000 to $32,000 installed. Soil quality moves this more than anything else — on tight clay requiring a mound, budget two to three times the low end.
How far must a Oregon drainfield be from a well?
100 feet from a private water well, with the septic tank itself allowed at 50 feet. Local rules can be stricter.
Sources
Every figure on this page traces to one of these documents.
- OAR 340-071-0220 — Standard Subsurface Systems · verified 2026-09-18
- OAR 340-071 — Onsite Wastewater Treatment Systems — rules and statutes · verified 2026-09-27