Washington · WAC 246-272A-0232
Septic system requirements in Washington
Tank sizes, drainfield rules, setbacks, permits, and real costs — pulled from Washington's own code rather than a national average.
- 3-bedroom tank
- 1,000 gal
- Design flow
- 120 gpd/BR
- Typical install
- $14,000–$36,000
- Permit fee
- $400–$1,500
- Well setback
- 100 ft
- Permitted by
- County / local
Septic tank sizes required in Washington
Washington sizes systems on a design flow of 120 gallons per day per bedroom, with every dwelling rated for at least 2 bedrooms. 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 | 240 gpd | 1,000 gal | 1,000 gal |
| 2 bedrooms | 240 gpd | 1,000 gal | 1,000 gal |
| 3 bedrooms | 360 gpd | 1,000 gal | 1,000 gal |
| 4 bedrooms | 480 gpd | 1,000 gal | 1,000 gal |
| 5 bedrooms | 600 gpd | 1,250 gal | 1,250 gal |
| 6 bedrooms | 720 gpd | 1,500 gal | 1,500 gal |
Source: WAC 246-272A-0232. 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 Washington at 360 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 | 450 ft² | 900 ft² | $11,700 |
| Sandy loam | 0.5 gpd/ft² | 720 ft² | 1,440 ft² | $14,300 |
| Loam | 0.4 gpd/ft² | 900 ft² | 1,800 ft² | $16,100 |
| Silt loam | 0.3 gpd/ft² | 1,200 ft² | 2,400 ft² | $19,000 |
| Clay loam | 0.2 gpd/ft² | 1,800 ft² | 3,600 ft² | $24,800 |
Estimates use Washington's cost index of 1.08× the national average. Build a line-item estimate →
Permits and who issues them
Local health jurisdictions permit systems under WAC 246-272A; DOH sets the standards and registers proprietary products.
| Permit required | Yes |
|---|---|
| Typical fee | $400 – $1,500 |
| Soil evaluation | Required before permit |
| Licensed installer | Required |
| Turnaround | 3–8 weeks |
| Ongoing inspection | Annual or every-three-year inspection required; Puget Sound counties enforce this actively |
| 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 | 5 ft |
| Surface water | 100 ft |
| Building foundation | 10 ft |
Regulatory contact
Washington State Department of Health — Wastewater Management
doh.wa.gov · 360-236-3330
What makes septic hard in Washington
Puget Sound counties require regular inspections and enforce them, so deferred maintenance surfaces quickly and expensively.
Glacial till west of the Cascades has a hardpan layer that perches water and rules out deep trenches.
Marine shoreline setbacks and critical-area rules restrict siting on exactly the lots people most want to build on.
Things that catch people out
- Washington requires a recurring inspection — annually for most complex systems, every three years for gravity systems. Skipping it can block a sale.
- Tank capacity is 250 gallons per bedroom with a 1,000 gallon floor, so a five-bedroom home needs 1,250 gallons.
- Two compartments are mandatory, with the first at one-half to two-thirds of total volume.
Washington septic questions
What size septic tank do I need in Washington?
A three-bedroom home in Washington requires a minimum 1,000 gallon tank under WAC 246-272A-0232. Design flow is 120 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 Washington?
Yes. Local health jurisdictions permit systems under WAC 246-272A; DOH sets the standards and registers proprietary products. Fees typically run $400 to $1,500, with a turnaround of about 3–8 weeks. A soil evaluation or percolation test is required before a permit is issued.
How much does a septic system cost in Washington?
A conventional system for a three-bedroom home typically runs $14,000 to $36,000 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 Washington 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.
- WAC 246-272A-0232 — Design requirements — Septic tank sizing · verified 2026-09-18
- WAC 246-272A — On-Site Sewage Systems · verified 2026-09-18