As 2026 extension season continues, several important federal filing deadlines are approaching. Below is a summary of the key dates, what they cover, and the most common pitfalls to flag with clients.
Key Federal Deadlines
| Deadline | Applies To | Form |
|---|---|---|
| September 15, 2026 | Extended calendar-year partnership and S corporation returns | Form 1065, Form 1120-S |
| September 15, 2026 | Third-quarter 2026 individual/business estimated tax payment | Form 1040-ES / 1120-W |
| September 30, 2026 | Extended calendar-year trust and estate returns | Form 1041 |
| October 15, 2026 | Extended individual returns (filed Form 4868 by April 15) | Form 1040 |
| October 15, 2026 | Extended calendar-year C corporation returns (filed Form 7004) | Form 1120 |
| October 15, 2026 | Final SEP IRA, SIMPLE IRA, and solo 401(k) contributions for 2025 (for filers who extended) | — |
| November 16, 2026 | Extended calendar-year exempt organization returns | Form 990, 990-EZ, 990-PF, 990-T |
Fiscal-year filers should calculate their own due dates based on their year-end; the table above assumes a calendar-year taxpayer.
The Distinction Clients Often Miss: Filing vs. Paying
An extension moves the filing deadline — it does not move the payment deadline. Any 2025 tax liability was due April 15, 2026, and interest has been accruing on unpaid balances since that date regardless of extension status. Clients who still owe should be encouraged to pay as much as possible now, both to stop interest from compounding and to reduce exposure to the failure-to-pay penalty.
Penalties for Missing an Extended Deadline
- Failure-to-file penalty: 5% of unpaid tax per month (or partial month), capped at 25%
- Failure-to-pay penalty: 0.5% of unpaid tax per month, also capped at 25%
- Interest: Compounds daily on any unpaid balance at the federal short-term rate plus 3%
There is no further extension available after the extended deadline passes for individuals — late returns after October 15 are simply late.
Connecticut Considerations
Connecticut generally conforms its individual and business extension deadlines to the federal schedule, but clients should not assume automatic conformity in every case — some CT-specific extension forms and payment requirements apply separately from the federal Form 4868/7004 process. Confirm current-year CT DRS guidance for each client rather than relying on the federal timeline alone.
Suggested Next Steps for Clients
- Confirm which entities/returns in the pipeline fall under the September 15 vs. October 15 deadlines.
- For anyone with a remaining balance, prioritize payment now over waiting until the extended filing date.
- Flag any SEP IRA / solo 401(k) contributions that need to be finalized by October 15 for extended filers.
- Verify Connecticut-specific extension and payment requirements alongside the federal deadline for each client.
Questions About Your Deadline?
Extension rules can vary depending on entity type, fiscal year, and state filing requirements, so it’s easy for details to fall through the cracks during a busy filing season. If you’re unsure which deadline applies to you, whether a payment is still owed, or what documentation you still need to pull together, reach out to your service team at Accavallo & Company, we’re here to help you sort it out before the date arrives, not after.
Sources to Verify
- IRS.gov — Form 4868 and Form 7004 instructions; IRS Publication 509
- Connecticut Department of Revenue Services (CT DRS) — current extension guidance
- Internal Revenue Code §6081 (extensions), §6651 (failure-to-file/pay penalties), §6601 (interest)
