File On Time, Fix It Later
If your S corporation or partnership return is on extension, it's due September 15. That's the end of the road. There is no second extension.
Most years, some clients get to this week with books that aren't finished. A bank account never got reconciled. A K-1 from another partnership hasn't shown up. Somebody is still hunting for the closing statement on a property sale.
The instinct is to wait until it's right. That instinct is expensive, and there's a better move.
What lateness actually costs
The late-filing penalty for a partnership lives in IRC §6698. For an S corporation it's §6699. They work the same way.
The penalty is a flat dollar amount per owner, per month, and it does not care how much tax is involved. A partnership that owes nothing still gets hit.
For returns required to be filed in 2026, that amount is $255 per partner or shareholder, per month, under Rev. Proc. 2024-40 (sections 2.56 and 2.57).
Two details make this worse than it sounds.
First, "month" means any part of a month. File on September 16 and you've triggered a full month. A two-shareholder S corp owes $510 for being one day late.
Second, the penalty runs up to 12 months. That same two-shareholder S corp, a year late, is looking at $6,120. A four-partner LLC in the same spot is at $12,240.
Get it filed on time and that number is zero.
Why filing on time works even when the return isn't perfect
Read §6698(a) closely and you'll see the penalty attaches to a partnership that "fails to file" the return "at the time prescribed." The trigger is the calendar. It is not accuracy.
So here's the move. If your books aren't finished by tomorrow, file anyway, using the best numbers you have. Then amend once the missing pieces arrive.
File wrong and on time, and you can generally correct it without a late-filing penalty. File right and late, and the penalty already ran and there's usually nothing to undo it.
That's the whole trade, and it's why the amended return is one of the most underused tools in a small business owner's kit.
The limit you need to respect
I want to be straight with you about where this stops working, because the statute has a second prong.
§6698(a) also applies to a partnership that files a return "which fails to show the information required." A return that's substantially blank isn't a get-out-of-jail card. It's an incomplete return, and it's exposed.
The courts use the Beard test to decide whether a document even counts as a return. From Beard v. Commissioner, 82 T.C. 766, 777 (1984), aff'd per curiam, 793 F.2d 139 (6th Cir. 1986), there are four elements:
There must be sufficient data to calculate tax liability
The document must purport to be a return
There must be an honest and reasonable attempt to satisfy the requirements of the tax law
The taxpayer must execute the return under penalties of perjury
Element three is the one that matters here. An honest and reasonable attempt is a real return, built from real books, with your best estimate where a number is genuinely unknown. It is not a placeholder with zeros in it.
In practice this is an easy bar to clear. If your accountant can produce a Profit & Loss and a Balance Sheet, you can file a return that satisfies Beard. You just may have to revise it later.
How you actually fix it, by entity type
This is where people get tripped up, because the correction mechanism depends on what kind of entity you run.
S corporations are simple. You file a corrected Form 1120-S with the amended return box checked, and you issue amended K-1s to the shareholders. No special procedure.
Partnerships are where it gets interesting. Under the Bipartisan Budget Act, a BBA partnership cannot file an amended Form 1065 after the due date. It has to file an Administrative Adjustment Request instead.
For an AAR, you file Form 8082 with a Form 1065 electronically, or Form 1065-X on paper, plus Forms 8985 and 8986 where the adjustments get pushed out to partners. You have three years from the later of the date you filed or the last day for filing, and you lose the option entirely once the IRS issues a Notice of Administrative Proceeding.
Partnerships that elected out of BBA under §6221(b) are back in the simple world and can file a straight amended return. That election is available to partnerships with 100 or fewer partners where every partner is an eligible type.
One more piece of timing. A superseding return is one you file before the due date, including extensions, and it simply replaces the original. Since September 15 is the extended due date, anything you send after that is an amended return or an AAR, not a superseding one.
Amend quickly, not eventually
There's a real reason to move fast rather than sit on it.
K-1s are information returns. A wrong one can draw a penalty under §6721 for the copy filed with the IRS and §6722 for the copy furnished to the owner. For 2026, that's up to $340 per K-1.
But those amounts drop sharply if you correct quickly. Fix it within 30 days and the penalty falls to $60. Fix it by August 1 and it's $130. Correct it promptly and, with reasonable cause, you may avoid it entirely.
The second reason is your own 1040. Individual returns on extension aren't due until October 15. If you amend the business return in the next few weeks, your owners receive corrected K-1s in time to file their personal returns once, with the right numbers, and never touch a 1040-X. Drag it past October 15 and every owner gets to amend too.
That's the difference between one cleanup and four.
Two more things worth knowing
If your e-filed return gets rejected, you are not automatically late. The IRS allows a transmission perfection period of 10 calendar days for business returns, and 5 days for extensions, to fix the rejection and resend. The original electronic postmark stands. This is exactly why you transmit in the morning and not at 11:58 at night.
Small partnerships have a reasonable cause backstop. Rev. Proc. 84-35 still applies after BBA, confirmed by the IRS in PMTA 2020-01. It covers a partnership with 10 or fewer partners, all of whom are individuals (other than nonresident aliens) or estates of deceased partners, where each partner's share of every item is allocated pro rata and each partner timely reported their share on their own return. It's reasonable cause relief, not an exemption from filing, and I would not plan around it. It's a safety net, not a strategy.
And the extension never extended payment. Filing on time protects you from §6698 and §6699. It does nothing about tax that was already due. Most S corps and partnerships owe nothing at the entity level, but an S corp can owe built-in gains tax under §1374 or excess net passive income tax under §1375, and those were due back in the spring. Your owners' personal payments were due April 15 too. If there's a balance, pay it at irs.gov/payments.
The short version
Reconcile what you can today. Tie the Profit & Loss and Balance Sheet to what the return will report. Get it to your accountant with hours to spare, not minutes.
And if the books still aren't done, file the best honest return you can build by tomorrow night. Then amend as soon as the missing pieces land, take every deduction you're owed, and get corrected K-1s into your owners' hands before October 15.
Perfect next week costs $255 an owner. Good tomorrow costs nothing.
I hope that helps.
Jonathan Sussman CPA
Sources
IRC §6698 and §6699 (late filing, partnerships and S corporations)
Rev. Proc. 2024-40, §§2.56, 2.57 (2026 filing-year penalty amount), and the §6721/§6722 information return amounts
Beard v. Commissioner, 82 T.C. 766, 777 (1984), aff'd per curiam, 793 F.2d 139 (6th Cir. 1986)
IRS, "File an administrative adjustment request for a BBA partnership"; Instructions for Forms 8082 and 1065-X
IRC §6221(b) (election out of BBA)
Rev. Proc. 84-35; PMTA 2020-01
IRS Pub. 4163 (e-file transmission perfection period)
IRC §1374, §1375 (S corporation entity-level taxes)
This post is general information, not advice for your situation. Tax rules change and the details matter. Talk to your CPA before you act on any of it.
