Illinois Sole Proprietorship Tax Deadlines 2026
Complete guide to Sole Proprietorship filing dates, extensions, estimated payment schedules, and required forms in Illinois for the 2026 tax year. Illinois imposes a state income tax, so Sole Proprietorship entities must file both federal and state returns.
Illinois Sole Proprietorship filing deadline
Business income reported on IL-1040.
Due: Apr 15, 2026
Extension: Oct 15, 2026
Form: Form IL-1040
Estimated payments
Quarterly estimated payment dates: Apr 15, 2026, Jun 15, 2026, Sep 15, 2026, Jan 15, 2027.
Due: Apr 15, 2026
Illinois Sole Proprietorship Tax Calendar at a Glance
| Event | Date |
|---|---|
| Filing deadline | Apr 15, 2026 |
| Extension deadline | Oct 15, 2026 |
| Q1 estimated payment | Apr 15, 2026 |
| Q2 estimated payment | Jun 15, 2026 |
| Q3 estimated payment | Sep 15, 2026 |
| Q4 estimated payment | Jan 15, 2027 |
FAQ
When is the Sole Proprietorship tax filing deadline in Illinois for 2026?
The Illinois Sole Proprietorship tax return is due Apr 15, 2026 for the 2026 tax year. An extension is available until Oct 15, 2026.
Does Illinois require Sole Proprietorship estimated tax payments?
Yes, Illinois requires estimated tax payments for Sole Proprietorship entities. The quarterly due dates are: Apr 15, 2026, Jun 15, 2026, Sep 15, 2026, Jan 15, 2027. Penalties may apply if you underpay or miss a deadline.
What form do I file for a Sole Proprietorship in Illinois?
Sole Proprietorship entities in Illinois file using Form Form IL-1040. You can download the form from Illinois Department of Revenue at https://tax.illinois.gov.
Can I get an extension for my Illinois Sole Proprietorship tax return?
Yes. Illinois allows an extension for Sole Proprietorship returns until Oct 15, 2026. Note that an extension gives you more time to file, not more time to pay — any tax owed is still due by the original deadline of Apr 15, 2026.
What are the penalties for filing late in Illinois?
Illinois typically charges a failure-to-file penalty (usually 5% of unpaid tax per month, up to 25%) and a failure-to-pay penalty (usually 0.5-1% per month). Interest also accrues on unpaid balances. Filing for an extension by the original deadline avoids the failure-to-file penalty but not the failure-to-pay penalty.
Where can I file Illinois Sole Proprietorship taxes?
File through Illinois Department of Revenue. Visit https://tax.illinois.gov for e-filing options, downloadable forms, and payment portals.
Illinois Estimated Tax Payments
View the full quarterly estimated payment schedule for Illinois, including all entity types and payment methods.
View Illinois estimated payments →