If you filed a Form 7004 for a calendar-year S-corp or partnership, September 15 is not a suggestion. It is the last day for the 1120-S or 1065.
Same date, different pile: Q3 estimated taxes for individuals and many calendar-year corporations.
People search "tax preparation" and "S-corp deadline" in the same week because the K-1 they need for their personal extension (October 15) is trapped inside a return nobody finished. That's not a software bug. That's a calendar.
An extension is extra time to file. It is not extra time to skip quarterly estimates. Shareholders still pay tax on their share of S-corp income on their 1040. The company return is how you get the K-1 that scores the game.
What is actually due
Calendar-year S-corp (Form 1120-S) on extension: due September 15, plus Schedule K-1 to each shareholder.
Calendar-year partnership (Form 1065) on extension: same idea. K-1s to partners.
Individuals: third quarter estimated tax (typically 1040-ES) for the current year.
If you never extended, you already had a March 15 (or next business day) problem. This post is for the people who did it right in the spring and then went silent.
Example: Two-owner S-corp, both on personal extension until October. Books close in August in a panic. K-1s go out September 14. One owner underpaid estimates all year because "the corp will handle it." The corp is a pass-through. The IRS still wants the owner's estimates. April already happened. September is the catch-up that doesn't fully catch up.
Not knowing your numbers is like driving without GPS. Filing 1120-S without a real P&L is how you amend in November.
Extension vs "I'll pay later"
| Factor | Form 7004 extension | What people think it is |
|---|---|---|
| Time to file the business return | Yes, to September 15 | Forever |
| Time to pay owner-level tax | No | "The extension covers my 1040 money" |
| K-1s | Still required with the return | Optional PDF whenever |
| Late 1120-S / 1065 | Per-shareholder / per-partner monthly penalties can stack | "It's just a form" |
| Q3 estimates | Still due September 15 | "I'll wait for the K-1" |
Having a tax advisor is like having a specialist instead of a general doctor. September is when general-purpose "we'll get to it" shops drown. If your books aren't closed, start with the monthly system. You cannot invent a year of coding in a weekend and call it a K-1.
What to do this week
- Close the books through a recent month. Bank recs. Owner pay split: reasonable salary vs distributions.
- Confirm 1120-S / 1065 is actually in progress, not "in the portal."
- Pay Q3 estimates from a real forecast, not last year's photocopy, if profit changed. See year-round planning.
- Tell shareholders when K-1s will exist. Their October 15 personal extension is downstream of your September 15.
- Don't use this week to invent Section 179 theater. Placed-in-service rules still care about the tax year that already ended for a 2025 return.
Under $400K, this is still worth doing clean. Late-filing penalties on an S-corp are not "advanced strategy." They're a leak. Over $400K, the K-1 also feeds QBI, basis, and next year's estimates. Messy September becomes an expensive April.
The short version
- September 15: extended calendar-year 1120-S and 1065, plus Q3 estimates.
- Extension ≠ payment holiday for the owners.
- K-1s unlock the personal return. No K-1, no honest 1040.
- Per-owner late-filing penalties can add up fast.
- Books first. Then the form.
FAQs
When is the S-corp extension deadline?
For a calendar-year S-corp that timely filed Form 7004, the 1120-S is generally due September 15. Fiscal-year entities use a different six-month date.
Does the extension delay my estimated taxes?
No. Quarterly estimates follow their own calendar. September 15 is also a standard Q3 date.
What if I miss September 15 for 1120-S?
Late-filing penalties can apply per shareholder per month. File as soon as you can and talk to your advisor about the notice. Don't ghost it.
Can I file 1120-S without K-1s?
The return includes the K-1s (or you're not done). Shareholders need those forms for their 1040s.
I'm a single-member LLC. Is this my deadline?
A disregarded SMLLC is not an S-corp. If you elected S-corp, you are in 1120-S land. If you didn't, this date may not be your entity deadline. Know your election.
What about October 15?
That's the usual extended individual 1040 date. It is not a second chance for the 1120-S that was due in September.
References
- IRS — S Corporations
- IRS — Form 1120-S
- IRS — Form 7004
- IRS — Estimated taxes
- IRS — About Schedule K-1 (Form 1120-S)
What to do next
Staring at an extended S-corp and a calendar that says September?
At CEOHAVEN, we help entrepreneurs and real estate investors with tax planning, tax preparation, and bookkeeping. You should know your numbers before the K-1 goes out, not after the penalty letter.
Book a call. We'll look at the books, the 1120-S, and the estimates that are due the same day.
It's not about how much you make. It's about how much you keep.