Illinois Business Tax Extension 2026: 2025 Tax-Year Filing Guide
LLC & Partnership · S Corporation · C Corporation · Fiduciary
This guide explains how to file an Illinois (IL) business tax extension for each entity type. For the federal filing process, see our IRS Form 7004 filing guide. Illinois grants automatic filing extensions for business income-tax returns; no extension request is required when no payment is due.
Need a pro? Connect with a U.S.-based CPA or EA to prepare and file your IL return.
Remember, payment is still due by the original deadline. In other words, extensions only extend filing time.
Illinois (IL) rules depend on entity type. Next, confirm the entity’s classification, file the required extension or retain federal proof when applicable, and pay required tax by the original deadline.
Illinois LLC Tax Extension: Filing Details
For a 2025 calendar-year filer, the extended filing deadline is October 15, 2026. Follow the Illinois return and extension method shown below.
In addition, the extension applies to filing time only. Therefore, pay required Illinois tax by the original deadline to limit penalties and interest.
The extension applies to filing time only. Pay required Illinois tax by the original deadline to limit penalties and interest.
Illinois S Corporation Tax Extension: Filing Details
For a 2025 calendar-year filer, the extended filing deadline is October 15, 2026. Follow the Illinois return and extension method shown below.
Illinois C Corporation Tax Extension: Filing Details
For a 2025 calendar-year filer, the extended filing deadline is November 16, 2026. Follow the Illinois return and extension method shown below.
The extension applies to filing time only. Pay required Illinois tax by the original deadline to limit penalties and interest.
The extension applies to filing time only. Pay required Illinois tax by the original deadline to limit penalties and interest.
Fiduciary, Estate, and Trust Filing Details
For a 2025 calendar-year filer, the extended filing deadline is October 15, 2026. Follow the Illinois return and extension method shown below.
Quick Reference: Deadlines and Forms
The table below provides a quick reference to Illinois business tax extensions.
| Entity Type | IL Return Form | Extension Method | Extended Deadline |
|---|---|---|---|
| LLC / Partnership | Form IL-1065 | Federal 5-month; Form IL-1065-V / Form IL-1065-V | Oct. 15, 2026 |
| S Corporation | Form IL-1120-ST | Form IL-1120-ST-V | Oct. 15, 2026 |
| C Corporation | Form IL-1120 / Form IL-1120-V | Form IL-1120-V | Nov. 16, 2026 |
| Fiduciary / Estate / Trust | Form IL-1041 | Form IL-1041 | Oct. 15, 2026 |
Frequently Asked Questions
Quick answers about Illinois business tax extensions – but always confirm the current rules with the Illinois Department of Revenue.
Visit Illinois Department of RevenueYes. However, the applicable entity-specific requirements listed above must be satisfied. Illinois grants automatic filing extensions, but the period and payment requirements depend on the return. If you need an individual rather than business extension, see our Illinois individual tax extension page.
Generally no. In other words, pay required tax by the original deadline and confirm the current state payment rules. An extension of time to file does not extend the time to pay – failing to pay by the original deadline can result in penalties and interest, even if the return is filed within the extended window.
No. In practice, partnerships, S corporations, C corporations, and fiduciaries can use different original dates, extension periods, or forms. For 2025 calendar-year returns filed in 2026, partnerships, LLCs taxed as partnerships, and fiduciaries generally receive a six-month extension to October 15, 2026. S corporations generally receive a seven-month extension to October 15, 2026, while C corporations generally receive a seven-month extension to November 16, 2026 for 2025 calendar-year returns.
Review the Illinois Department of Revenue business tax guidance for the entity’s return. The Department publishes current return forms, payment vouchers, and any changes to extension periods or payment requirements.
Estimated reading time: 6 minutes