Septic system requirements in King County, Washington
The septic rules landplan.io applies in King County, what a septic tank costs there, and the parcel data we hold.
Updated
Design a septic system for an address in King County
Washington's state rules apply in King County: landplan.io has no county rules of its own here. State rules: Reviewed See Washington's rules
Septic systems allowed in King County
The system types landplan.io permits in King County:
- 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 King County
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 King County
| 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 King County
| 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 King County. Confirm a design with the Washington health department or your county's before you build.
Parcel data in King County
landplan.io has parcel boundaries for King County (99.9% of its area): type an address and the design starts from the parcel.
Septic companies, designers and jobs in Washington
- Septic companies and distributors in Washington
- Septic designers in Washington
- Septic design jobs in Washington
Where the rules come from
The regulations and documents landplan.io's King County, Washington rules are read from:
Near King County
- Pierce County
- Snohomish County
- Kitsap County
- Island County
- Kittitas County
- Thurston County
- Chelan County
- Mason County
- All of Washington
Questions
What septic systems are allowed in King County?
landplan.io designs by Washington's state rules there and permits: 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 King County?
landplan.io sizes the septic tank in King County 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 King County?
landplan.io keeps these distances in King County: 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 for King County, Washington?
Yes: parcel boundaries cover King County (99.9% of its area).