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Illinois Business State Tax Extension I C Corporations Filing Guide

IL-1120-V | Payment Voucher for Corporation Income and Replacement Tax  ·  C Corporations  ·  Business Filing Guide

Overview

Illinois C Corporations are subject to state corporate income tax requirements, administered by the Illinois Department of Revenue. Illinois grants an automatic extension tied to an approved IRS extension, making the process simple for businesses that have already filed IRS Form 7004.

Original Filing Deadline

• The original due date for the Illinois C Corporations return is April 15, 2026. File or remit full payment by this date to remain in good standing.
• Extension Period & Extended Deadline
• Illinois automatically grants an extension to file, provided you have obtained an approved federal extension from the IRS by filing IRS Form 7004. With a valid IRS extension, the Illinois extended deadline becomes October 15, 2026. No separate state extension form is required—the federal extension serves as your Illinois extension.

Payment Is Still Due by Original Deadline

The extension applies to filing only—it is not an extension to pay taxes owed. Any outstanding tax liability must be paid by the original due date of April 15, 2026 to avoid penalties and interest. Use the Illinois | Online Tax Extension Payment to pay electronically. For mail-in payments, use IL-1120-V | Payment Voucher for Corporation Income and Replacement Tax and send to: Illinois Department of Revenue P.O. Box 19038 Springfield, IL 62794-9038

Late Filing and Payment Penalties

• Failure to File: Late filing: Lesser of $250 or 2% of tax liability, reduced by timely payments or credits. Nonfiling: Greater of $250 or 2% of tax liability up to $5,000, if a return is not filed within 30 days after receiving a notice of nonfiling.
• Failure to Pay: Before audit: 2% of tax liability for payments 1-30 days late. 10% of tax liability for payments 31 or more days late. After audit initiation: 15% of tax liability if paid within 30 days after issuance of the audit-prepared amended return or waiver of restrictions on assessment. 20% of tax liability if taxpayer: paid after the expiration of the 30-day period; paid under protest; or claims a credit or refund for tax, penalties, or interest determined due upon audit.

Best Practices

• Estimate and pay your Illinois business tax by April 15, 2026 to avoid interest and penalties.
• File IRS Form 7004 with the IRS to secure your federal extension, which Illinois automatically honors.
• Use the state online payment portal for fast, convenient electronic payment.
• Set a reminder to file your completed Illinois C Corporations return by October 15, 2026.

Final Thoughts

• Illinois’s business tax extension is straightforward once your federal extension is approved. Pay 100% of any tax due by April 15, 2026 and file your C Corporations return by October 15, 2026.
• Quick Reference Table
• State Abbreviation
• IL
• State Name
• Illinois
• Business Entity Type
• C Corporations
• Extension Status
• Approved with Federal Extension
• Return Due Date
• April 15, 2026
• Extended Due Date
• October 15, 2026
• Other Information
• Illinois grants your business an automatic 6-month extension provided you have obtained an approved IRS extension.
• The extension is NOT an extension to pay any taxes you may have due.
• Tax payments are due by the original return due date.
• Online Payment
• Illinois | Online Tax Extension Payment
• Payment Voucher Form
• IL-1120-V | Payment Voucher for Corporation Income and Replacement Tax
• State Payment Agency
• Illinois Department of Revenue
• Mailing Address with Payment
• Illinois Department of Revenue P.O. Box 19038 Springfield, IL 62794-9038
• Failure to File Penalties
• Late filing: Lesser of $250 or 2% of tax liability, reduced by timely payments or credits. Nonfiling: Greater of $250 or 2% of tax liability up to $5,000, if a return is not filed within 30 days after receiving a notice of nonfiling.
• Failure to Pay Penalties
• Before audit: 2% of tax liability for payments 1-30 days late. 10% of tax liability for payments 31 or more days late. After audit initiation: 15% of tax liability if paid within 30 days after issuance of the audit-prepared amended return or waiver of restrictions on assessment. 20% of tax liability if taxpayer: paid after the expiration of the 30-day period; paid under protest; or claims a credit or refund for tax, penalties, or interest determined due upon audit.

Working with a Tax Professional

Business tax returns are complex—a licensed CPA or Enrolled Agent can maximize deductions, ensure accurate calculations, and handle filing on your behalf. Here's how working with a TaxPro typically goes:

1. 1Get matched with a verified U.S.-based CPA or Enrolled Agent
2. 2Upload your business documents via secure drag-and-drop
3. 3Review and approve your return with upfront, transparent fees
4. 4Your TaxPro files your Illinois return by October 15, 2026

Quick Reference Table

State Abbreviation	IL
State Name	Illinois
Business Entity Type	C Corporations
Extension Status	Approved with Federal Extension
Return Due Date	April 15, 2026
Extended Due Date	October 15, 2026
Other Information	Illinois grants your business an automatic 6-month extension provided you have obtained an approved IRS extension. The extension is NOT an extension to pay any taxes you may have due. Tax payments are due by the original return due date.
Online Payment	Illinois | Online Tax Extension Payment
Payment Voucher Form	IL-1120-V | Payment Voucher for Corporation Income and Replacement Tax
State Payment Agency	Illinois Department of Revenue
Mailing Address with Payment	Illinois Department of Revenue P.O. Box 19038 Springfield, IL 62794-9038
Failure to File Penalties	Late filing: Lesser of $250 or 2% of tax liability, reduced by timely payments or credits. Nonfiling: Greater of $250 or 2% of tax liability up to $5,000, if a return is not filed within 30 days after receiving a notice of nonfiling.
Failure to Pay Penalties	Before audit: 2% of tax liability for payments 1-30 days late. 10% of tax liability for payments 31 or more days late. After audit initiation: 15% of tax liability if paid within 30 days after issuance of the audit-prepared amended return or waiver of restrictions on assessment. 20% of tax liability if taxpayer: paid after the expiration of the 30-day period; paid under protest; or claims a credit or refund for tax, penalties, or interest determined due upon audit.