If your S corporation or partnership filed an extension back in March, Tuesday, September 15, 2026 is the end of the line. There is no second extension. And because the late-filing penalty on these returns is charged per owner, per month rather than as a percentage of tax owed, a partnership that owes nothing can still walk away with a four-figure bill.
What’s actually due September 15
The IRS third-quarter calendar puts four things on that Tuesday that matter to a small business:
- Form 1120-S — calendar-year S corporations that timely requested a 6-month extension.
- Form 1065 — calendar-year partnerships, which includes most multi-member LLCs, on the same 6-month extension.
- Third 2026 estimated tax installment for individuals (Form 1040-ES) — the one that catches owners whose K-1 income has no withholding behind it.
- Third 2026 estimated tax deposit for corporations, via EFTPS.
The penalty is per owner, per month
This is the part that surprises people. For returns whose original due date fell after December 31, 2025 — which covers every calendar-year 2025 return that was due March 16, 2026 — the IRS failure-to-file penalty is $255 per partner or shareholder, per month or partial month, for up to 12 months. A three-shareholder S corp two months late owes $1,530 before anyone looks at whether tax was due. California stacks its own on top: $18 per partner, member, or shareholder, per month, also up to 12 months.
Reasonable cause relief applies to both returns. And partnerships with 10 or fewer partners — all individuals or estates, income allocated proportionally, every partner reporting their share on a timely filed personal return — are presumed to have reasonable cause under Rev. Proc. 84-35. Presumed is not automatic; you still have to meet every condition.
California doesn’t use the same date for everything
Here is where LA County owners trip. The FTB’s extended due dates are not a copy of the federal ones:
- S corporation (Form 100S) — extended due date is the 15th day of the 9th month, so September 15. Same as federal.
- Partnerships and LLCs taxed as partnerships (Forms 565 / 568) — extended due date is the 15th day of the 10th month, so October 15. A month later than federal.
That extra California month is real, and it is also a trap: your federal Form 1065 is still due September 15 no matter what the state calendar says. File federal first, then use October for the state return if you need it.
One more quirk on the same date: the FTB weights estimated payments 30% — 40% — 0% — 30%, so the September 15 state installment is zero — only the federal payment moves. We covered that in the June estimates article, and every date for the year sits in our 2026 California deadline calendar.
If your books for 2025 still aren’t closed, you have about three weeks — and the cost of missing this date scales with how many owners your entity has, not with how profitable it was. A four-partner LLC that files three months late owes $3,060 in federal penalty on a return that may report no tax at all. That is a fee for paperwork, and it is entirely avoidable.
Before September 15
- Confirm your March extension was actually filed and accepted — don’t assume
- Get 2025 books closed and to your CPA this week, not the weekend before
- Check your owner count — that number is the penalty multiplier
- Pay the third federal estimate (Form 1040-ES); California’s September installment is 0%
- Calendar October 15 for California Forms 565/568 and extended personal 1040s
- If you know you’ll be late, file anyway — partial months count, so one day late costs the same as one month late
This article is general information, not tax advice for your specific situation. Rules change and details matter — talk to a CPA (we know one) before acting on anything here.