Your personal return is due October 15. The Schedule K-1 it depends on is not due until September 15.
Thirty days. And the form you are waiting on is one you do not control.
That gap is where small business tax preparation happens in the fall. Not in the software, not in the receipts. In the dependency chain between one return and the next.
If you extended back in March or April, you did not buy six quiet months. You bought a deadline with a queue in front of it. And a balance that has been growing since spring.
An extension moves the filing date, never the payment date. The IRS is direct about it: "The extension is only for filing your return." Tax owed was due at the original deadline. Since then, two separate charges have been running on any unpaid balance. One is a failure-to-pay penalty of 0.5% of the unpaid tax per month, capped at 25%. The other is interest that compounds daily at a rate the IRS resets every quarter. September 15, 2026 carries two obligations at once for pass-through owners. Those are the extended Form 1065 or 1120-S, and the third-quarter estimated payment. October 15, 2026 is the extended date for individual returns and calendar-year C corporations.
Owners extend for one of two reasons. Either a K-1 from somebody else's entity never showed up, or the money to cover the bill was not there.
In April those two feel identical. You file the form, the deadline moves, the pressure lifts. By August they are not remotely the same situation. The difference decides what the rest of this year costs you.
The document problem is a scheduling problem. It resolves the moment the paperwork lands. The only real risk is running out of calendar.
The money problem never paused. It has been growing quietly since spring, on a schedule most owners have never looked at.
Sorting out which one you have is the first honest step in fall tax preparation. Almost everyone assumes they have the first. A meaningful number have both.
The meter never stopped in April
Most owners read an extension as a pause. It is permission to file late, and nothing else.
The IRS says so in one sentence on its own extension page: the extension is only for filing your return.
If you owed money in April and did not pay it, two things have been happening every day since.
The first is the failure-to-pay penalty. It runs at 0.5% of the unpaid tax for each month or part of a month. The maximum is 25%. Paying on the 2nd costs the same as paying on the 28th.
The second is interest. It compounds daily. The rate is not fixed for the year. The IRS sets it every quarter.
Look at what it did in 2026:
| Quarter | Underpayment rate (non-corporate) |
|---|---|
| Q1 2026 (Jan through Mar) | 7% |
| Q2 2026 (Apr through Jun) | 6% |
| Q3 2026 (Jul through Sep) | 7% |
| Q4 2026 (Oct through Dec) | Check the IRS quarterly interest rates page for the current figure |
A balance carried from April 15 to October 15 crosses three different rate periods. The last of them was still unpublished when this was written. The "I will just pay it when I file" plan cannot be priced in advance. You are not paying a number you can look up. You are paying a number that is still being written.
You do not have to wait for the return to be finished to stop the bleeding. Both the penalty and the interest are calculated against the balance. A payment applied to the account reduces that balance, starting the day it posts. Paying something in August is strictly better than paying everything in October. Designate the payment to the right tax year and form, through Direct Pay, EFTPS or your extension voucher, so it lands on the correct balance.
What is due on September 15, 2026?
Two different obligations land on the same Tuesday.
The extended return. A calendar-year partnership that extended in March files Form 1065 on September 15. So does an S corporation with Form 1120-S.
The third-quarter estimated payment. That is a personal obligation. The owner owes it on income earned in 2026.
One is last year's paperwork. The other is this year's money. Owners handle one and forget the other. The calendar shows a single entry.
Missing the filing side is expensive. Partnerships and S corporations usually owe no tax at the entity level. The penalty is not based on tax owed. It is based on how many owners you have.
The Form 1065 instructions set the 2026 figure at $255 for each month or part of a month the return is late. The amount is adjusted for inflation most years. You multiply that by the number of partners, for up to 12 months. Form 1120-S works the same way, counting shareholders.
Three partners, two months late, no tax owed at the entity level. The arithmetic is $1,530.
Missing the payment side works differently. The charge for a short estimated payment is interest under section 6654, as we covered in Q3 Estimated Tax 2026. It is computed separately on each installment, not a flat fine.
Why your October 15 return depends on a September 15 form
If you own part of a partnership or an S corporation, your personal return cannot be finished until that entity issues your Schedule K-1. The K-1 reports your share of the business income. It flows straight onto your 1040.
The entity's K-1 is due when the entity's return is due, extensions included. An entity that uses its full extension delivers your K-1 on September 15.
Your extended 1040 is due October 15.
Thirty days to receive a document, review it, resolve anything that looks wrong, and prepare a return around it. That is why "I will get to it in October" fails so reliably.
It also compounds. Say you hold an interest in an entity. That entity holds an interest in another one. The K-1 at the bottom of that chain has to be issued before the one above it can be finalized. Each layer consumes days out of the same 30.
Do not treat September 15 as the target for the entity return. Treat it as the ceiling. Every day the 1065 or 1120-S closes early adds a day to the window for the individual returns that depend on it. For a multi-owner business, finishing the entity return in late August does not help the entity much. It hands every owner three extra weeks.
What small business tax preparation requires now
The January checklist assumes you are starting cold. This one assumes you started in the spring, stopped, and are picking it back up.
- The extension confirmation itself, showing you filed it before the original deadline
- Proof of what you paid with the extension, and the date it posted
- Every estimated tax payment made since, with confirmation numbers
- Books reconciled through December 31 of the tax year, not through today
- K-1s received from any partnership or S corporation you hold an interest in
- A written list of which K-1s are still outstanding and who owes them to you
- Any corrected 1099s or amended statements that arrived after your first attempt
- Notes on what stalled the return in the spring, so the same question does not stall it again
- Asset purchases made during the tax year, which drive depreciation and first-year write-off decisions
- A current-year profit estimate, which you need for the September 15 estimated payment anyway
You build one number to size your Q3 estimated payment. That same number tells you whether this year is tracking above or below last year.
What each missed date costs
| What you miss | What it triggers |
|---|---|
| Payment that was due at the original deadline | 0.5% per month of unpaid tax, max 25%, plus daily compounding interest |
| Extended Form 1065 or 1120-S on September 15 | $255 per partner or shareholder, per month, up to 12 months |
| Q3 estimated payment on September 15 | Interest under section 6654, computed per installment |
| Extended Form 1040 or 1120 on October 15 | 5% per month of unpaid tax, max 25%, reduced to 4.5% in any month the failure-to-pay penalty also applies |
Two details in that last row matter. The failure-to-file penalty is ten times the failure-to-pay penalty. Filing on time while still owing money is always better than doing neither. And when both apply in the same month, the IRS does not stack them at 5.5%. It reduces the filing penalty so the combined charge is 5%.
There is also a floor. It applies to returns due after December 31, 2025. An individual or C corporation return filed more than 60 days late carries a minimum failure-to-file penalty. It is $525, or 100% of the tax owed, whichever is less. October 15 plus 60 days lands in mid-December. A return that slips into the new year is no longer being penalized as a percentage of a small balance. It is being penalized against that floor.
There is also relief on the payment side. If you get an installment agreement approved, the failure-to-pay penalty drops to 0.25% per month while the plan is in good standing. Left alone long enough to draw a notice of intent to levy, it climbs to 1%.
The relief rule changed in July, and it lands on this return
If you have been filing and paying on time for years, the way you get penalty relief changed.
On July 8, 2026, the IRS announced Automatic Exemption from Penalty, or AEP. It replaces First Time Abate, the program most business owners and their accountants have used for the last decade.
The change that matters is in the first word.
Under First Time Abate, relief was something you asked for. Someone had to notice the penalty and know the program existed. Plenty of eligible taxpayers paid instead, because nobody told them there was a door.
Under AEP, the IRS states it plainly: "Taxpayers do not need to take action to receive this relief."
| Automatic Exemption from Penalty (AEP) | |
|---|---|
| Who qualifies | A history of timely filing and paying in the three prior years, or 12 consecutive quarters for quarterly returns |
| Penalties covered | Failure to file, failure to pay, and failure to deposit |
| Returns the IRS names as eligible | Forms 1040, 1065 and 1120, plus the employment tax series. Form 1120-S is absent from that list |
| What you must do | Nothing. Relief is applied automatically |
| When it starts | Summer 2026, for eligible original returns beginning with tax year 2025 |
| What it replaces | First Time Abate, phasing out through summer 2026 |
AEP begins with tax year 2025 original returns, the ones now waiting on September 15 and October 15.
The eligible-returns row deserves a second look. The IRS names Forms 1040, 1065 and 1120, and Form 1120-S is absent. Its list of covered penalties, though, includes section 6699. That is the S corporation late filing penalty. The agency has not reconciled the two lists. Partnerships filing Form 1065 are clearly reached. S corporation owners should confirm eligibility with their preparer rather than assume it.
AEP addresses penalties. It does not erase the interest on unpaid tax. A clean compliance history may take the failure-to-pay penalty off a late balance. The daily compounding interest on that same balance continues untouched. Relief is a reason to file, never a reason to leave tax unpaid.
The transition completes for returns with original due dates on or after January 1, 2027. Next spring's filings sit fully inside the new system.
Florida has no personal state income tax. Sole proprietors and owners of pass-through entities generally have no state return chasing these federal dates. C corporations are the exception once they have nexus in the state. Nexus means a connection strong enough to create a filing duty, such as an office, employees or property here. Those companies file Form F-1120. They pay the state corporate income tax of 5.5% on net income above the $50,000 exemption. A federal extension does not settle a state obligation, so confirm the Florida side separately.
The only part of this you control
Nearly everything above runs on its own. The interest compounds whether or not you check the rate. The penalty counts a partial month as a whole one whether or not you know the date. The relief either applies to your record or it does not.
One thing is still yours to move. The thirty days.
The gap between the September 15 K-1 and the October 15 return is not fixed by law. It is fixed by when the entity return gets finished. Close the 1065 or the 1120-S in late August and every owner downstream gets those three extra weeks. Close it on the deadline and they get a month, minus however long the mail and the questions take.
That is the whole lever. Not the forms, not the software. The order you do them in.
Frequently Asked Questions
No. The IRS states plainly that the extension is only for filing your return. Any tax owed was due at the original deadline. Since that date, the unpaid balance has been carrying a failure-to-pay penalty of 0.5% per month and daily compounding interest.
You can file the individual return using a good-faith estimate built on the entity's preliminary numbers, then amend once the K-1 arrives. Document how you reached the estimate. A materially wrong figure carries its own accuracy-related exposure, so do this with your preparer. Filing something by October 15 avoids the 5% per month failure-to-file penalty. That is the larger of the two exposures. For anyone who still owes tax, waiting silently is the choice most likely to cost the most.
Generally no. The six-month extension is the full amount available to most business and individual filers. That makes October 15 a hard stop rather than a checkpoint. Limited exceptions exist for taxpayers in federally declared disaster areas and for certain filers living abroad.
Yes. The late filing penalty for a partnership is not tied to tax owed. It is $255 per partner for each month or part of a month the return is late. The cap is 12 months. A partnership with no entity-level tax liability can still accumulate a meaningful penalty by being late with paperwork.
Not the way you used to. In July 2026 the IRS introduced Automatic Exemption from Penalty. It replaces First Time Abate and applies relief without a request. Eligibility rests on a history of timely filing and payment in the three prior years, or 12 consecutive quarters for quarterly returns. It covers the failure-to-file, failure-to-pay and failure-to-deposit penalties. It begins with tax year 2025 original returns, so it reaches returns filed on extension in 2026. One caveat matters here. The IRS names Forms 1040, 1065 and 1120 as eligible and omits Form 1120-S, while still listing the S corporation penalty under section 6699 as covered. Because AEP is new and that gap is unresolved, confirm your own eligibility with your CPA before relying on it. It addresses penalties only. It does not remove interest on unpaid tax.
Yes. Both the failure-to-pay penalty and the interest are calculated against the unpaid balance. Any payment that posts reduces the base both charges run on, starting that day. You do not need a completed return to make a payment toward the account.
Sources
- Internal Revenue Service. Get an extension to file your tax return. IRS.gov
- Internal Revenue Service. Failure to Pay Penalty. IRS.gov
- Internal Revenue Service. Failure to File Penalty. IRS.gov
- Internal Revenue Service. Quarterly interest rates. IRS.gov
- Internal Revenue Service. IR-2026-83, IRS simplifies penalty relief, introduces automatic process for eligible taxpayers. July 8, 2026.
- Internal Revenue Service. Instructions for Form 1065, Penalties: Late Filing of Return. IRS.gov
- Internal Revenue Service. Instructions for Form 1120-S, Penalties. IRS.gov
- Internal Revenue Code section 6654, Failure by individual to pay estimated income tax.
- Florida Department of Revenue. Corporate Income Tax Filing Requirements. FloridaRevenue.com
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