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Most people with an IRS payment plan or a pending Offer in Compromise treat September 15 like a routine estimated tax date. That is the wrong first question. The right first question is whether missing that payment will default the plan you already have.
The third quarterly estimated tax payment for 2026 is due September 15, 2026. Form 1040-ES sets that date. For calendar year individuals, the payment covers income from June 1 through August 31.
If you are self employed, on 1099 income, or running light on withholding, this is not just a penalty problem. Under IRC 6159 and the Installment Agreement rules, you must stay current on filing and current year deposits. Under the Offer in Compromise rules, the same compliance condition applies while the offer is pending. Miss the estimate, and the agreement you fought to get can unwind.
Key Takeaways
- The 2026 third estimated tax installment is due September 15, 2026 under Form 1040-ES.
- Underpayment of estimated tax is a separate addition under IRC 6654. That is not the whole story if you already have a collection case.
- An installment agreement under IRC 6159 can default if you fail to pay another tax liability when it is due, including current year tax that becomes due.
- A pending Offer in Compromise can be returned if you fail to remain compliant with estimated tax payments or deposits. A return generally has no appeal rights.
- Currently Not Collectible status still expects timely estimated payments and deposits in many cases. New unpaid liabilities can restart collection.
- If you cannot pay the full September 15 amount, contact a representative before the deadline. Waiting until the default notice arrives narrows your options.
What September 15 Actually Is
Estimated tax is how the IRS collects tax on income that is not fully withheld. Self employment earnings, gig work, interest, dividends, rents, and similar income usually fall here. Form 1040-ES is the worksheet and voucher package for individuals.
For calendar year taxpayers, the four due dates are April 15, June 15, September 15, and January 15 of the following year. IRC 6654(c) sets those installment dates in statute. The September 15 payment is installment three.
Generally, you must make estimated payments if you expect to owe at least $1,000 after withholding and refundable credits, and your withholding falls short of the safe harbor. The usual safe harbor is the lesser of 90% of the current year tax or 100% of last year’s tax. If prior year adjusted gross income exceeded $150,000, that prior year floor rises to 110%. Those rules live in IRC 6654(d) and Form 1040-ES.
This post is not a deep dive on the underpayment penalty. We already covered that. The point here is different. If you are already in collection, the September 15 payment is a compliance checkpoint.
The Mistake We See on Payment Plans
Most taxpayers assume an installment agreement only fails if they miss the monthly draft. They are wrong about the full default risk.
IRC 6159(b)(4) lets the IRS alter, modify, or terminate an agreement if you fail to pay any installment when due, fail to pay any other tax liability when due, or fail to provide requested financial updates. “Any other tax liability when due” is the trap for people who stay current on the plan payment and skip current year estimates.
IRM 5.14.1 is blunt. Before an agreement is approved, individuals must be current on estimated tax payments or withholding. Business taxpayers with employees must be current on federal tax deposits. During the agreement, current returns must be filed and current deposits paid, or the agreement defaults. Revenue officers are told to remind taxpayers that missing estimates can create new liabilities that default the plan.
IRM 5.14.11 lists the default grounds in practice. Failure to pay another tax liability when it is due is an independent reason to propose termination. That includes related TINs for the same taxpayer. When the IRS proposes default, you typically receive CP 523 or Letter 2975. You generally have 30 days to cure before termination. Levy restrictions continue for a defined period after the notice, but the clock is real.
We see this pattern constantly with freelancers and business owners. The monthly installment clears. April arrives with a new balance due. Or September 15 passes with no estimated payment. The system flags the account. Suddenly the “safe” plan is in default status.
If your plan is already tight, do not wait for CP 523. Ask whether you can restructure before a new unpaid balance posts. Staying current on estimates is often cheaper than reinstating after default.
What a Missed Estimate Does to an Offer in Compromise
An Offer in Compromise under IRC 7122 lets you settle for less than the full balance when the IRS agrees full collection is unrealistic. Eligibility starts with filing compliance and required estimated payments. The IRS OIC page is clear: you must have filed required returns and made required estimated payments. Employers must also have made tax deposits for the current and prior two quarters before applying.
That duty does not end at submission. While the offer is pending, you must remain current. IRM 5.8.7.2.2.2 requires return of a processable offer if you fail to verify sufficient estimated tax or withholding, or if you fail to remain compliant after submission. The combo letter often warns that missed quarterly estimates will cause return without further consideration. The quarterly dates listed in that IRM language are April 15, June 15, September 15, and January 15.
Returned and rejected are not the same outcome. A returned offer generally has no appeal rights. The IRS keeps the application fee on a processable return. A rejected offer can be appealed within 30 days on Form 13711. Late in the investigation, after a preliminary determination, noncompliance can lead to rejection with appeal rights instead of return. Either way, the offer you spent months building can die because a quarterly estimate was skipped.
We have seen strong Doubt as to Collectibility packages collapse for one reason: the taxpayer stayed focused on the old balance and ignored current year deposits. The RCP math never got a fair hearing because compliance failed first.
For a fuller look at what the IRS measures on eligibility, see our post on what the IRS looks at for OIC eligibility. This post is about the September 15 landmine specifically.
Currently Not Collectible Is Not a Free Pass
Currently Not Collectible status pauses enforced collection when the IRS agrees you cannot cover basic living expenses and the tax at the same time. It does not erase the debt. Interest and penalties continue. Refunds can still be offset. Liens may still be filed.
TAS guidance is direct. While you apply for CNC, the IRS will require you to continue making estimated tax payments and federal tax deposits on time. After CNC hardship, later federal tax liabilities can remove you from uncollectible status and restart levy risk.
IRM 5.16.1 is especially strict for in business CNC closed with closing code 13. The taxpayer must be current on filing and paying requirements, including estimated tax payments when applicable, and must show an ability to remain current. Noncompliance can reactivate the case.
If you are in CNC and September 15 is approaching, treat the estimate as part of keeping that pause intact.
If You Cannot Pay the Full Amount on September 15
Cash shortfalls are common in the third quarter. That does not mean the only options are full payment or silence.
First, figure what you actually owe for the quarter. Use the Form 1040-ES worksheet or Pub. 505. If income was uneven, the annualized income installment method under IRC 6654(d)(2) may lower the required September installment. That method belongs on Form 2210 when you file. It is not a free pass for collection compliance, but it can reduce an artificial shortfall.
Second, pay what you can before the deadline. Partial estimated payments still post. They reduce underpayment exposure under IRC 6654 and show current year effort on a collection account.
Third, increase withholding if you have wages or a pension. Withholding is treated as paid evenly through the year under IRC 6654(g). A late year Form W-4 change can repair some of the gap without a separate September voucher.
Fourth, if you have an installment agreement or pending OIC, call your representative before September 15. Ask whether the IRS has already warned you about estimated tax shortfalls. Ask whether a combo letter deadline is open. Ask whether restructuring the IA now is safer than waiting for default.
Fifth, do not skip filing later to “catch up.” Filing compliance and payment compliance are separate. Unfiled returns create their own problems under the IA and OIC rules.
What Happens If You Already Missed It
If September 15 has passed and the payment did not go in, move fast.
Check your Individual Online Account for estimated tax credits and any default indicators. Look for CP 523 or Letter 2975 if you have an installment agreement. Look for any OIC correspondence demanding estimated tax catch up.
If the IA is in default but not yet terminated, cure within the notice window when you can. IRM 5.14.11.5 says agreements in default must be reinstated if the taxpayer remedies the default, unless another default reason remains. Reinstatement often requires proof you are now current on estimates and deposits.
If an OIC combo letter gave you a short deadline, usually around 15 days, meet it or document why the requested amount is wrong. Provide a current year estimate calculation if your required payment is lower than the prior year based figure.
If levy risk is already live, read our IRS tax levy pages and act on any Final Notice of Intent to Levy before the 30 day window closes. A CDP hearing may still be available depending on the notice. A defaulted plan can reopen enforced collection. That is how a missed estimate turns into a frozen bank account.
The Bottom Line
September 15 is not only an estimated tax date. For anyone in an installment agreement, a pending Offer in Compromise, or CNC, it is a compliance gate.
The underpayment penalty under IRC 6654 is real. The larger risk for taxpayers already in crisis is losing the collection alternative that was holding levies back. IRC 6159 and the OIC program both demand current year compliance. The Internal Revenue Manual tells IRS employees to enforce that demand.
If you cannot pay the full September 15 estimate, do not ignore the date. Calculate what is required, pay what you can, fix withholding, and get representation before a default or return letter issues.
Matthew Wildes, JD, CPA | CLAW Tax Group / Wildes At Law
Schedule a consultation or call (651) 323-2255.
The content of this article is for informational purposes only and does not constitute legal or tax advice. Consult a qualified tax professional for guidance specific to your situation.
