Washington · WAC 246-272A
The Washington State Department of Health writes the rule and local health jurisdictions administer it, with the twelve Puget Sound counties operating under enhanced requirements.
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.
Glacial till with a hardpan layer across the Puget Sound lowlands, outwash sand in places, volcanic soils in the Cascades, and loess in the Palouse. Winter rainfall keeps water tables high on the west side.
Conventional gravity systems where soil allows, but pressure distribution, sand filters and mounds are extremely common on till and hardpan sites. Treatment standards are tiered by soil type.
| Dwelling | Design flow | Minimum tank | With disposal |
|---|---|---|---|
| 1 bedroom | 240 gpd | 1,000 gal | 1,250 gal |
| 2 bedrooms | 240 gpd | 1,000 gal | 1,250 gal |
| 3 bedrooms | 360 gpd | 1,000 gal | 1,250 gal |
| 4 bedrooms | 480 gpd | 1,250 gal | 1,500 gal |
| 5 bedrooms | 600 gpd | 1,500 gal | 1,750 gal |
Washington requires an operation and maintenance program in the Puget Sound counties, including periodic inspections at intervals set by system type and reporting to the local health jurisdiction. Property owners are responsible for keeping the system inspected.
The state sets a floor. These Washington counties are where the largest septic populations sit, and where local requirements most often exceed it.
| County | What makes it different |
|---|---|
| King | Seattle fringe; till and hardpan, strict O&M requirements |
| Pierce | Large septic population on glacial soils |
| Snohomish | Growth with high winter water table |
| Kitsap | Peninsula with extensive on-site use and marine water protection |
| Thurston | Olympia area, groundwater protection focus |
| Clark | Southwest Washington growth on mixed soils |
| Spokane | Eastern Washington; aquifer sensitive area rules |
A three-bedroom home at 120 gallons per bedroom produces 360 gallons per day of design flow. On sandy loam that needs roughly 600 square feet of absorption area. On clay loam it rises to about 1,800 square feet, and on clay a conventional field is usually not permitted at all. Run your own soil class through the drainfield calculator.
Puget Sound counties require periodic inspection — commonly annually for advanced systems and every three years for gravity systems — with reports filed to the health jurisdiction. Use the pump schedule calculator to work out the interval for your tank size and household.
Regional labour and permit costs in Washington run about 122% of the national baseline. Here is what typical installations land at for a three-bedroom home:
| System type | Typical installed cost in Washington | When it applies |
|---|---|---|
| Conventional gravity | $11,000 – $19,500 | Good soil, adequate depth to water |
| Chamber system | $12,100 – $22,400 | Alternative to gravel trenches |
| Mound system | $20,900 – $41,000 | Shallow water table or thin soil |
| Aerobic treatment | $17,600 – $35,100 | Clay soil or small lots |
| Tank pump-out (service) | $390 – $790 | 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 WAC 246-272A, On-Site Sewage Systems. County and health district rules routinely exceed them, code text changes, and only Washington State DOH — Wastewater Management Section or your local health department can confirm what applies to your parcel.
Send your sizing numbers to installers who work under Washington rules every day.