While federal deadlines for C‑Corporations (Form 1120) and S‑Corporations (Form 1120‑S) are well‑known—April 15 and March 15 respectively—each state sets its own due dates and extension rules. Failure to align state filings with federal timelines can result in penalties, interest, and lost extension privileges. This guide provides a detailed comparison for major jurisdictions in the 2025–26 season.
1. Federal Filing Deadlines & Extensions
- C‑Corporations: File Form 1120 by April 15; extension to October 15 via Form 7004.
- S‑Corporations: File Form 1120‑S by March 15; extension to September 15 via Form 7004.
- Note: Extensions apply to filing only, not payment. Tax due must accompany the extension request to avoid failure‑to‑pay penalties.
2. California (CA)
California conforms closely to federal dates but requires separate state filings:
- C‑Corporations (Form 100): Due April 15; extension to October 15 via Form 3539 with payment of 90% of estimated CA tax.
- S‑Corporations (Form 100S): Due March 15; extension to September 15 via Form 3539 and 90% payment requirement.
- Late payment or underpayment triggers interest and 5%–25% late‑filing penalties.
3. New York (NY)
New York State and New York City allow federal extensions but impose payment thresholds:
- C‑Corporations (Form CT‑3/CT‑3‑A): Due April 15; extension to October 15 by attaching federal Form 7004 and paying 100% of prior‑year or 90% of current‑year NY tax.
- S‑Corporations (Form CT‑3‑S): Due March 15; extension to September 15 via Form CT‑5 and similar payment rules.
- NYC Business Tax: Separate Form EXT‑02 required for city‑level extension.
4. Florida (FL)
Florida has no personal income tax but does tax corporations:
- C‑Corporations (Form F‑1120): Due May 1 (for calendar-year); extension to November 1 via Form F‑1120EXT with payment of estimated tax.
- S‑Corporations: No separate state return—S‑Corp income flows through to shareholder returns (no personal state tax).
- Late filings incur 10% penalty on unpaid tax plus interest at 4.79% (2025 rate).
5. Texas (TX)
Texas imposes a franchise tax on “taxable entities” rather than traditional corporate income tax:
- Franchise Tax Report: Due May 15; extension to November 15 via Form 05‑164 with payment of 90% of the “No‑Tax‑Due” threshold.
- Both C‑Corps and S‑Corps file the same franchise report; S‑Corp income remains exempt from personal state tax.
- Penalties for late filing begin at 5% of tax due plus interest at 1% per month.
6. Illinois (IL)
Illinois requires separate state returns for both entity types:
- C‑Corporations (Form IL‑1120): Due April 15; extension to October 15 via Form 700‑ES with payment of 50% of prior‑year tax liability.
- S‑Corporations (Form IL‑1120‑ST): Due March 15; extension to September 15 via Form 505‑ES and 50% payment rule.
- Late filings incur 1% per month penalty (capped at 36%) plus interest at the adjustable statutory rate.
7. Comparison Table
| State | C‑Corp Original/Extended | S‑Corp Original/Extended | Extension Form & Payment |
|---|---|---|---|
| Federal | Apr 15 / Oct 15 | Mar 15 / Sep 15 | Form 7004; pay estimate |
| California | Apr 15 / Oct 15 | Mar 15 / Sep 15 | Form 3539; 90% CA tax |
| New York | Apr 15 / Oct 15 | Mar 15 / Sep 15 | Attach Fed Form 7004/CT‑5; 100% or 90% |
| Florida | May 1 / Nov 1 | — | Form F‑1120EXT; estimate |
| Texas | May 15 / Nov 15 | May 15 / Nov 15 | Form 05‑164; 90% franchise |
| Illinois | Apr 15 / Oct 15 | Mar 15 / Sep 15 | Form 700‑ES/505‑ES; 50% prior‑yr |
8. Best Practices for 2025–26
- Master Calendar: Consolidate federal and state dates with reminders.
- Estimate Early: Calculate state and federal liabilities by Q1 to fund extensions.
- Coordinate Filings: File state extension forms concurrently with federal extensions to ensure alignment.
- Track Payments: Retain proof of extension filings and estimated tax remittances.
- Review Changes: Monitor state revenue department updates each quarter.
9. Conclusion
State deadlines for C‑Corporations and S‑Corporations can vary widely from federal timelines. By understanding each jurisdiction’s original due dates, extension procedures, and payment requirements, you can avoid costly penalties and ensure a smooth filing season for 2025–26.
