Illinois pairs its income tax with the Personal Property Replacement Tax and grants unusually long automatic extensions. Two of its deadlines diverge from the federal calendar — partnerships file in April, and corporations extend to November — so mirroring the federal dates produces errors.
Illinois Tax Deadlines at a Glance (2026)
Entity Type | Return Due | Extended Due Date |
|---|---|---|
Individuals (IL-1040) | April 15, 2026 | October 15, 2026 (6-month) |
C Corporations (IL-1120) | April 15, 2026 | November 15, 2026 (7-month) |
S Corporations (IL-1120-ST) | March 16, 2026 | October 15, 2026 (7-month) |
Partnerships (IL-1065) | April 15, 2026 | October 15, 2026 (6-month) |
Note the divergences: C corporations extend to November 15 (a 7-month automatic extension), not October 15. And partnerships (IL-1065) are due April 15 — the 15th day of the 4th month — not the federal March 16. Both S-corp and partnership returns then extend to October 15. Illinois does not follow the federal March 15 partnership deadline.
The Personal Property Replacement Tax
On top of income tax, Illinois imposes the Replacement Tax on entity net income: 2.5% for C corporations and 1.5% for S corporations, partnerships, and trusts. Combined with the 7% corporate income tax, a C corporation’s total Illinois rate is 9.5%. The individual income tax is a flat 4.95%.
Illinois PTE Elective Tax
Illinois offers an elective pass-through entity tax at 4.95% of net income, letting partnerships and S corporations pay at the entity level so owners receive a matching credit — a SALT-cap workaround. The election is made annually on the IL-1065 or IL-1120-ST. Its original sunset was removed by 2025 legislation, so it continues indefinitely for 2026 and beyond.
Estimates, Extensions, and Penalties
Illinois grants its automatic extensions with no form required, but they extend time to file only — use IL-505-I (individuals) or IL-505-B (businesses) to pay by the original date. Individual estimated payments are due April 15, June 15, September 15, 2026, and January 15, 2027. The late-payment penalty is 2% (1–30 days) or 10% (31+ days); the 2026 underpayment interest rate is 7%.
CPA Action Steps
Extend C-corps to November 15 (7-month), not October 15
File IL-1065 partnerships in April, not on the federal March date
Layer the 1.5%/2.5% Replacement Tax onto the income tax
Consider the 4.95% PTE election — now permanent
Verify current rates and dates against the Illinois Department of Revenue (tax.illinois.gov) before filing.