Illinois · Illinois Private Sewage Disposal Licensing Act and Code
The Illinois Department of Public Health writes the code and licenses installers; most counties administer permitting through local health departments.
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, generally tight silty clay loam with slow percolation. Southern Illinois has claypan soils that are among the more difficult in the Midwest. Sandy areas appear along river valleys.
Conventional trenches where percolation allows. Aerobic treatment units with surface discharge are permitted in Illinois in circumstances many states prohibit, which is a genuine local difference. Mounds and sand filters are used on tight soils.
| 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 |
Illinois licenses private sewage disposal contractors, and the code governs both design and installation. Surface discharge of treated effluent to a ditch or waterway is allowed under specific conditions with an NPDES-type permit — unusual, and worth confirming locally.
The state sets a floor. These Illinois counties are where the largest septic populations sit, and where local requirements most often exceed it.
| County | What makes it different |
|---|---|
| Cook | Mostly sewered; on-site systems in outlying townships |
| Will | Suburban growth on tight soils |
| McHenry | Large septic population, groundwater protection focus |
| Lake | High-value lots, strict local requirements |
| Kane | Mixed suburban and agricultural |
| Madison | Southern claypan soils |
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.
Aerobic units with surface discharge carry sampling and maintenance obligations. Use the pump schedule calculator to work out the interval for your tank size and household.
Regional labour and permit costs in Illinois run about 106% of the national baseline. Here is what typical installations land at for a three-bedroom home:
| System type | Typical installed cost in Illinois | When it applies |
|---|---|---|
| Conventional gravity | $9,500 – $17,000 | Good soil, adequate depth to water |
| Chamber system | $10,400 – $19,600 | Alternative to gravel trenches |
| Mound system | $18,000 – $35,700 | Shallow water table or thin soil |
| Aerobic treatment | $15,200 – $30,600 | Clay soil or small lots |
| Tank pump-out (service) | $340 – $690 | 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 Illinois Private Sewage Disposal Licensing Act and Code, 77 Illinois Administrative Code 905. County and health district rules routinely exceed them, code text changes, and only Illinois DPH — Private Sewage Disposal Program or your local health department can confirm what applies to your parcel.
Send your sizing numbers to installers who work under Illinois rules every day.