Septic system requirements in Washington
What landplan.io applies when it designs a septic system in Washington: the system types allowed, tank sizes, setbacks and soil loading rates, with what septic products cost there.
Updated
Design a septic system for an address in Washington
Reviewed landplan.io designs by Washington's own rules, which our staff reviewed against the state's regulations. Last published 5 October 2026.
Septic systems allowed in Washington
The system types landplan.io permits in Washington:
- Mounded Pressure System
- Drip-Line Dispersal
- SSAS Pressure Trenches
- SSAS Gravity/Flood Dosed Trenches
- Bed System
- Oscar Subsurface Drip
- Discharging System
Septic tank size in Washington
The daily design flow of a home: 120 gal/day per bedroom, at least 240 gal/day. The septic tank is sized from that flow:
| Daily design flow | Minimum tank |
|---|---|
| up to 360 gal/day | 900 gallons |
| 360 to 480 gal/day | 1,000 gallons |
| 480 to 600 gal/day | 1,250 gallons |
| 600 to 720 gal/day | 1,500 gallons |
| 720 to 840 gal/day | 1,750 gallons |
| 840 to 960 gal/day | 2,000 gallons |
| 960 to 1,080 gal/day | 2,250 gallons |
| 1,080 to 1,200 gal/day | 2,500 gallons |
| 1,200 to 1,320 gal/day | 2,750 gallons |
| 1,320 to 1,440 gal/day | 3,000 gallons |
Septic setbacks in Washington
| From | To | Distance |
|---|---|---|
| LineString | Down-gradient cuts or banks with at least 5 ft. of original, undisturbed soil above a restrictive layer due to a structural or textural change | 25 ft |
| LineString | Down-gradient cuts or banks with less than 5 ft. of original, undisturbed soil above a restrictive layer due to a structural or textural change | 50 ft |
| LineString | Down-gradient Interceptor/curtain drains/foundation drains/ drainage ditches | 30 ft |
| LineString | Down-gradient Other site features that may allow effluent to surface | 30 ft |
| LineString | Other adjacent soil dispersal components/ subsurface stormwater infiltration systems | 10 ft |
| LineString | Pressurized water supply line | 10 ft |
| LineString | Up-gradient Interceptor/curtain drains/foundation drains/ drainage ditches | 10 ft |
| LineString | Up-gradient Other site features that may allow effluent to surface | 10 ft |
| Point | Decommissioned well (decommissioned in accordance with chapter 173-160 WAC) | 10 ft |
| Point | Public drinking water spring measured from the ordinary high-water mark | 200 ft |
| Point | Public drinking water well | 100 ft |
| Point | Spring or surface water used as drinking water source measured from the ordinary high-water mark | 100 ft |
| Point | Well or suction line | 100 ft |
| Polygon | Building foundation/in-ground swimming pool | 10 ft |
| Polygon | Existing Septic Component to be Abandoned | 5 ft |
| Polygon | Property or easement line | 5 ft |
| Polygon | Surface water measured from the ordinary high-water mark | 100 ft |
Soil loading rates in Washington
| Table | Lowest | Highest | Soil groups |
|---|---|---|---|
| Soil Loading Rate | 0.2 gal/day/ft² | 1 gal/day/ft² | 6 |
landplan.io picks the rate from the soil test: texture, structure and percolation.
These are the values the landplan.io design engine applies in Washington. Confirm a design with the Washington health department or your county's before you build.
Septic system costs in Washington
| Product | Median | Typical range | Listings |
|---|---|---|---|
| Dose Tank | $1,675 | $1,556 – $1,736 | 32 |
| Septic Tank | $1,822 | $1,789 – $1,943 | 45 |
The Washington price index: every product type, month by month.
Parcel data in Washington
landplan.io has parcel boundaries for 37 of Washington's 39 counties, and part of 2 more: the counties where 97% of the state's people live. Type an address there and the design starts from the parcel.
Septic companies, designers and jobs in Washington
- Septic companies and distributors in Washington (92)
- Septic designers in Washington
- Septic design jobs in Washington
Where the rules come from
The regulations and documents landplan.io's Washington rules are read from:
Septic rules by county in Washington
- Adams County
- Asotin County
- Benton County
- Chelan County
- Clallam County
- Clark County
- Columbia County
- Cowlitz County
- Douglas County
- Ferry County
- Franklin County
- Garfield County
- Grant County
- Grays Harbor County
- Island County
- Jefferson County
- King County
- Kitsap County
- Kittitas County
- Klickitat County
- Lewis County
- Lincoln County
- Mason County
- Okanogan County
- Pacific County
- Pend Oreille County
- Pierce County
- San Juan County
- Skagit County
- Skamania County
- Snohomish County
- Spokane County
- Stevens County
- Thurston County
- Wahkiakum County
- Walla Walla County
- Whatcom County
- Whitman County
- Yakima County
Questions
What septic systems are allowed in Washington?
landplan.io permits these system types in Washington: Mounded Pressure System, Drip-Line Dispersal, SSAS Pressure Trenches, SSAS Gravity/Flood Dosed Trenches, Bed System, Oscar Subsurface Drip and Discharging System.
What size septic tank do I need in Washington?
landplan.io sizes the septic tank in Washington from the daily design flow: up to 360 gal/day: 900 gallons; 360 to 480 gal/day: 1,000 gallons; 480 to 600 gal/day: 1,250 gallons; larger flows need larger tanks. A home's design flow is 120 gal/day per bedroom, at least 240 gal/day.
How far must a septic system be from a well in Washington?
landplan.io keeps these distances in Washington: Point to public drinking water well: 100 ft; Point to well or suction line: 100 ft; Point to decommissioned well (decommissioned in accordance with chapter 173-160 wac): 10 ft.
Does landplan.io have parcel data in Washington?
Yes, for 37 of Washington's 39 counties, and part of 2 more.