Wisconsin · Wisconsin Administrative Code SPS 383
The Department of Safety and Professional Services writes SPS 383 and counties administer permitting through licensed professionals.
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 across most of the state with a seasonal high water table, karst limestone in the northeast around Door County and the Kettle Moraine, and sandy soils in the central sands region.
Wisconsin's POWTS framework recognises multiple system categories. Mounds and at-grade systems are extremely common because of shallow limiting conditions, and Wisconsin pioneered mound design in the United States.
| Dwelling | Design flow | Minimum tank | With disposal |
|---|---|---|---|
| 1 bedroom | 300 gpd | 1,000 gal | 1,250 gal |
| 2 bedrooms | 300 gpd | 1,000 gal | 1,250 gal |
| 3 bedrooms | 450 gpd | 1,000 gal | 1,250 gal |
| 4 bedrooms | 600 gpd | 1,250 gal | 1,500 gal |
| 5 bedrooms | 750 gpd | 1,500 gal | 1,750 gal |
Wisconsin requires a soil and site evaluation by a licensed Certified Soil Tester, and a design by a licensed plumber or POWTS designer. Counties run a maintenance program requiring periodic pumping or inspection reporting — commonly on a three-year cycle.
The state sets a floor. These Wisconsin counties are where the largest septic populations sit, and where local requirements most often exceed it.
| County | What makes it different |
|---|---|
| Waukesha | Milwaukee suburbs; till soils and groundwater protection |
| Dane | Madison area, growth on till |
| Door | Karst limestone with severe constraints and strict rules |
| Sauk | Mixed soils, tourism development |
| Marathon | Central Wisconsin, variable conditions |
| Oneida | Northwoods lake country with shoreline setbacks |
A three-bedroom home at 150 gallons per bedroom produces 450 gallons per day of design flow. On sandy loam that needs roughly 750 square feet of absorption area. On clay loam it rises to about 2,250 square feet, and on clay a conventional field is usually not permitted at all. Run your own soil class through the drainfield calculator.
Wisconsin counties operate a POWTS maintenance program, commonly requiring pumping or inspection every three years with a report filed to the county. Use the pump schedule calculator to work out the interval for your tank size and household.
Regional labour and permit costs in Wisconsin run about 102% of the national baseline. Here is what typical installations land at for a three-bedroom home:
| System type | Typical installed cost in Wisconsin | When it applies |
|---|---|---|
| Conventional gravity | $9,200 – $16,300 | Good soil, adequate depth to water |
| Chamber system | $10,100 – $18,700 | Alternative to gravel trenches |
| Mound system | $17,500 – $34,200 | Shallow water table or thin soil |
| Aerobic treatment | $14,700 – $29,300 | Clay soil or small lots |
| Tank pump-out (service) | $330 – $660 | 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 Wisconsin Administrative Code SPS 383, Private Onsite Wastewater Treatment Systems. County and health district rules routinely exceed them, code text changes, and only Wisconsin DSPS — Private Onsite Wastewater Treatment or your local health department can confirm what applies to your parcel.
Send your sizing numbers to installers who work under Wisconsin rules every day.