What happens if you miss the September 15 tax deadline?

September 15 is the extended filing deadline for calendar-year partnerships, S corporations, and multi-member LLCs taxed as either. Missing it starts two separate clocks: a failure-to-file penalty and a failure-to-pay penalty, even though the underlying tax was already due back in March. The situation is fixable. Filing as soon as possible stops the meter, and penalty relief through reasonable cause or first-time abatement is a real path worth asking about.

What is the September 15 tax deadline actually for?
What happens if you file an S-corp or partnership return after September 15?
Can you file another extension after September 15?
What happens if you missed the September 15 estimated tax payment instead?
How do you request penalty relief from the IRS?
Does missing September 15 affect your state filing obligations too?
What should you do first if you are already behind?
How do you keep this from happening again next year?
Key Takeaways
References

What is the September 15 tax deadline actually for?

September 15 is the extended due date for calendar-year Form 1065 partnerships and Form 1120S S corporations that filed Form 7004 for a six-month extension, according to TurboTax. The original deadline for those entities is March 15, so the extension moves the filing date, not the payment date.

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Pass-through entities do not pay federal income tax at the entity level, but the owners' individual returns and estimates still depend on the numbers on that return. September 15 is also the third-quarter estimated tax due date for individuals, which is a separate obligation from the entity return, according to Anderson Advisors. Dates shift year to year and should be confirmed for the current tax year.

What happens if you file an S-corp or partnership return after September 15?

The failure-to-file penalty is typically 5% of the unpaid tax for each month the return is late, capped at 25%, according to H&R Block. The failure-to-pay penalty runs at 0.5% per month on the unpaid amount.

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When both penalties apply, the IRS reduces the failure-to-file penalty by the failure-to-pay amount so the combined rate does not exceed 5% per month. If a return is more than 60 days late, the minimum penalty is the lesser of a set dollar amount or 100% of the tax due. Interest accrues daily at the federal short-term rate plus 3%, and it compounds on top of penalties, according to H&R Block.

Can you file another extension after September 15?

No. The IRS allows one six-month extension per tax year, and there is no second Form 7004 for the same return, according to H&R Block. October 15 is the final extended deadline for individual returns filed with Form 4868, but that does not reopen the entity deadline.

The practical answer is to file the return as soon as the numbers are ready rather than look for more time. Filing late but filing is almost always cheaper than continuing to wait, because the failure-to-file penalty is the larger of the two. The IRS states that an extension of time to file is not an extension of time to pay, according to IRS.

What happens if you missed the September 15 estimated tax payment instead?

Quarterly estimated payments for individuals are generally due January 15, April 15, June 15, and September 15, according to Block Advisors. Missing an estimate triggers an underpayment penalty rather than a failure-to-file penalty, because there is no return to file.

The penalty is calculated on the shortfall for the period, so catching up quickly reduces the damage. Owners of pass-through entities often miss the September estimate because they are focused on the entity return due the same day. Dates shift year to year and should be confirmed for the current tax year, according to Anderson Advisors.

How do you request penalty relief from the IRS?

Reasonable cause relief requires showing a legitimate reason for the late filing and that you acted with ordinary care once the problem was identified. First-time penalty abatement is a separate administrative waiver for taxpayers with a clean compliance history, and it is requested by phone or in writing, according to H&R Block.

The IRS does not grant relief automatically. Someone has to ask, and the request has to be documented. Getting current on the return itself is usually the first step, because relief is evaluated against a filed return. Spencer Accounting Group handles this work through Sales Tax Resolution and Back Tax Return Filing, sequencing the catch-up so compliance comes back with the least disruption.

Does missing September 15 affect your state filing obligations too?

Many states follow the federal extension automatically, but some do not and require their own extension request. State deadlines vary widely, and a handful of states set income tax deadlines on dates other than April 15, according to Anderson Advisors.

Residents of Alaska, Florida, Nevada, South Dakota, Texas, Washington, and Wyoming pay no state income tax, but that does not remove other state obligations. If you have nexus in multiple states, a missed federal deadline can cascade into several state late-filing situations at once. Confirm the current-year date for every state where you file, because these shift year to year, according to Block Advisors.

What should you do first if you are already behind?

File the return, even if you cannot pay the full balance, because the failure-to-file penalty is the steeper of the two. Set up a payment plan or partial payment to stop the failure-to-pay penalty from compounding, according to H&R Block.

Reconstruct the numbers from bank and platform records if the books are not current. Clean bookkeeping pays for itself here. Look at whether the same problem exists in prior years before filing only the current one. Ask about penalty relief rather than assuming it does not apply. The IRS states that filing as soon as possible limits the penalties that continue to accrue, according to IRS.

How do you keep this from happening again next year?

Put the March 15 and September 15 entity dates, the four estimate dates, and the April 15 individual date on one calendar. Reconcile the books monthly so the return is a filing exercise, not a reconstruction project. Build estimates off actual profit rather than last year's number, since pass-through income moves.

Track where you have nexus as you add states, because filing obligations follow sales and payroll, not just physical locations. Confirm each year's dates, since weekends and holidays shift them, according to TurboTax and IRS.

Key Takeaways

September 15 is the extended deadline for calendar-year partnership and S-corp returns, not the deadline to pay the tax.
The failure-to-file penalty is typically 5% per month up to 25%, and the failure-to-pay penalty is 0.5% per month.
The IRS allows only one six-month extension per tax year, so there is no second extension after September 15.
Interest accrues daily at the federal short-term rate plus 3% and is separate from penalties.
Reasonable cause relief and first-time penalty abatement both exist, but neither is automatic.
Missing the September 15 estimated payment is a different problem from missing the entity return, with a different penalty.
This article is general information, not tax advice for a specific situation; a consultation is the way to find out whether Spencer Accounting is a fit.