Illinois · 77 Ill. Adm. Code 905
Septic system requirements in Illinois
Tank sizes, drainfield rules, setbacks, permits, and real costs — pulled from Illinois's own code rather than a national average.
- 3-bedroom tank
- 1,000 gal
- Design flow
- 150 gpd/BR
- Typical install
- $9,000–$24,000
- Permit fee
- $150–$700
- Well setback
- 75 ft
- Permitted by
- County / local
Septic tank sizes required in Illinois
Illinois sizes systems on a design flow of 150 gallons per day per bedroom. The tank must meet both the bedroom table below and a 1.5× daily-flow detention requirement — whichever produces the larger number governs.
| Bedrooms | Design flow | Code minimum | Recommended |
|---|---|---|---|
| 1 bedroom | 150 gpd | 1,000 gal | 1,000 gal |
| 2 bedrooms | 300 gpd | 1,000 gal | 1,000 gal |
| 3 bedrooms | 450 gpd | 1,000 gal | 1,000 gal |
| 4 bedrooms | 600 gpd | 1,250 gal | 1,250 gal |
| 5 bedrooms | 750 gpd | 1,500 gal | 1,500 gal |
| 6 bedrooms | 900 gpd | 1,750 gal | 2,000 gal |
Source: 77 Ill. Adm. Code 905. 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 Illinois at 450 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 | 563 ft² | 1,126 ft² | $12,200 |
| Sandy loam | 0.5 gpd/ft² | 900 ft² | 1,800 ft² | $15,500 |
| Loam | 0.4 gpd/ft² | 1,125 ft² | 2,250 ft² | $17,700 |
| Silt loam | 0.3 gpd/ft² | 1,500 ft² | 3,000 ft² | $21,300 |
| Clay loam | 0.2 gpd/ft² | 2,250 ft² | 4,500 ft² | $28,600 |
Estimates use Illinois's cost index of 1.08× the national average. Build a line-item estimate →
Permits and who issues them
IDPH writes the Private Sewage Disposal Code and local health departments administer it; installers are licensed by the state.
| Permit required | Yes |
|---|---|
| Typical fee | $150 – $700 |
| Soil evaluation | Required before permit |
| Licensed installer | Required |
| Turnaround | 2–6 weeks |
| Ongoing inspection | Not mandated statewide |
| 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 | 75 ft |
|---|---|
| Well (septic tank) | 50 ft |
| Property line | 10 ft |
| Surface water | 50 ft |
| Building foundation | 10 ft |
Regulatory contact
Illinois Department of Public Health — Private Sewage Program
dph.illinois.gov · 217-782-5830
What makes septic hard in Illinois
Glacial till across central Illinois is dense silty clay with very slow percolation, which is why so many older systems were built with surface discharge and are now being replaced.
Seasonal high water tables in the flat prairie counties limit trench depth to almost nothing.
Illinois still has a legacy of surface-discharging systems that fail current standards and must be upgraded on repair.
Things that catch people out
- Surface-discharging systems require an NPDES permit and ongoing sampling — a meaningful recurring cost most homeowners do not expect.
- Tank capacity is the greater of the bedroom table or 1.5 times estimated daily flow, so a large household escapes the table quickly.
Bedroom-by-bedroom capacities live in Appendix A, Illustration F, which is published as a diagram. Confirm with your local health department.
Illinois septic questions
What size septic tank do I need in Illinois?
A three-bedroom home in Illinois requires a minimum 1,000 gallon tank under 77 Ill. Adm. Code 905. Design flow is 150 gallons per day per bedroom, and the tank must also hold 1.5 times daily flow, so the practical recommendation is 1,000 gallons.
Do I need a permit for a septic system in Illinois?
Yes. IDPH writes the Private Sewage Disposal Code and local health departments administer it; installers are licensed by the state. Fees typically run $150 to $700, with a turnaround of about 2–6 weeks. A soil evaluation or percolation test is required before a permit is issued.
How much does a septic system cost in Illinois?
A conventional system for a three-bedroom home typically runs $9,000 to $24,000 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 Illinois drainfield be from a well?
75 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.
- 77 Ill. Adm. Code 905 — Private Sewage Disposal Code · verified 2026-09-18
- 77 Ill. Adm. Code 905.40 — Septic Tanks · verified 2026-09-18