October 15 came and went. Your return is still sitting unfiled, and every day since has felt slightly worse than the one before.
Here is the part nobody tells you: a missed tax extension deadline is far less expensive to fix today than it is to keep avoiding. The penalty that hurts most is the one for not filing, not the one for not paying, and it runs at ten times the cost. Which means the return you have been putting off is the cheapest thing you can do this week.
No notice has arrived yet. That is the window.
This blog walks through what is accruing right now and what it adds up to: how the failure to file and failure to pay penalties stack, why interest keeps running after the penalties stop, what California adds on top, and which of those charges can still be removed. It closes with what to do this week if your return is still outstanding.
What Happens First After a Missed Tax Extension Deadline
In the short term, nothing visible. No phone call, no agent, no letter the following week.
What starts immediately is the accrual. Penalties and interest build quietly in the background, and the first you hear of it is usually a notice weeks after you file.
One point is worth clearing up, because it causes most of the confusion. The extension you filed in April only moved the filing deadline. It never moved the payment deadline. Any tax you owed was due on April 15, and the late payment charges have been running since then, extension or not.
If you are owed a refund, the picture is much calmer. There is no failure to file penalty when no tax is owed. You still want to file, and sooner than you think.
A refund claim generally has to be made within three years of filing the return or two years of paying the tax, whichever is later. For a return that was never filed, the practical window runs roughly three years from the original due date, and the extension you filed can affect how the lookback period is calculated. It is not a clean bright line, so it is worth checking rather than assuming the refund is still there.
A Missed Form 1040 Due Date and a Missed Entity Deadline Are Not the Same
October 15 was the extended due date for individual returns. Partnerships and S corporations had an earlier one, September 15, so an unfiled entity return is already a month further along than a personal one.
That gap matters more than it sounds. A missed Form 1040 due date starts one clock, calculated on the tax you owe. A missed partnership or S corporation deadline starts a different one, calculated per owner, which means it grows whether or not the entity owes a cent.
If you have both outstanding, deal with the entity return first.
How Much Is the Penalty for Filing Taxes Late?
The failure to file penalty is 5% of the unpaid tax for each month or part of a month the return is late, capped at 25%. Part of a month counts as a whole month, so filing on November 2 costs the same as filing on November 28.
There is also a floor. If the return is more than 60 days late, the minimum penalty is the lesser of $525 for returns required to be filed in 2026, or 100% of the tax owed.
Sixty days from October 15 lands in mid-December. Filing before that date avoids the minimum penalty entirely, which matters most for people who owe a small amount, where $525 can exceed what the percentage penalty would have been.
How the Failure to File and Failure to Pay Penalties Stack
The failure to pay penalty is 0.5% of the unpaid tax per month, also capped at 25%, and it has been running since April 15 rather than October 15.
When both apply in the same month, the failure to file penalty is reduced by the failure to pay amount, so the combined charge is 5% a month rather than 5.5%. The file penalty maxes out after five months. The pay penalty keeps going until the balance is cleared, and the two together top out at 47.5% of the tax.
The practical takeaway is the ratio. Failing to file costs ten times as much per month as failing to pay.
That is why the advice is always to file even when you cannot pay. Filing stops the 5% clock. It does not stop the 0.5% clock, but that is the cheaper of the two by a wide margin.
Two things change the pay penalty rate:
- An approved installment agreement cuts it from 0.5% to 0.25% per month, but it requires the return to be filed first.
- A Notice of Intent to Levy raises it from 0.5% to 1% per month, which is one reason ignoring that particular notice is costly.
IRS Late Filing Interest Runs Even After the Penalties Stop
Interest is separate from penalties and behaves differently. The rate is the federal short-term rate plus three percentage points, set quarterly by the IRS. For individual underpayments it ran at 7% in the first quarter of 2026, dropped to 6% in the second, and returned to 7% for both the third and fourth quarters.
Three features matter. It compounds daily, it runs from April 15 regardless of the extension, and it applies to the penalties as well as the tax, so the balance grows on top of itself.
Penalties have caps. Interest has none. That is why an old unfiled year can eventually exceed the tax that started it.
Interest is also the hardest charge to remove. The IRS abates it only where the charge came from its own error or unreasonable delay, not because you had a good reason for filing late.
California Adds a Second Set of Penalties
If you are in San Diego County, there are two agencies involved, and the Franchise Tax Board runs its own schedule.
- Individuals and businesses, late filing: 5% of the tax due per month, up to 25%, calculated from the original due date.
- Late payment: a one-time 5% charge plus 0.5% per month, capped at 40 months.
- Partnerships and LLCs treated as partnerships: $18 per partner or member, per month, for up to 12 months, which tops out at $216 per owner.
- S corporations: $18 per shareholder, per month, for up to 12 months, on the same terms.
- Demand penalty: if the FTB issues a demand to file and it is ignored, 25% of the total tax due, regardless of payments made on time.
Those per-owner penalties are the reason an unfiled entity return deserves faster attention than an unfiled personal one. A six-partner firm accrues $108 a month on the per-partner penalty alone, before the percentage charges are counted, and the balance owed has nothing to do with it.
One detail worth confirming before you assume any of this applies. An LLC taxed as a corporation falls outside the partnership rules and follows the corporate schedule instead, so how the entity is classified on the return changes which penalty you are facing.
California grants an automatic filing extension without a form, so there was nothing to miss on that side. The payment deadline was still April 15.
Can IRS Penalties Be Waived?
Penalties are more negotiable than most people assume. There are three routes.
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Automatic penalty relief:
Beginning in summer 2026, the IRS started moving eligible taxpayers off First Time Abate and onto an automatic process it calls the Automatic Exemption from Penalty, or AEP. Eligibility generally turns on timely filing and payment for the prior three years, or twelve consecutive quarters for quarterly filers, along with meeting the filing and payment requirements tied to the current balance. Where it applies, the IRS suppresses failure to file, failure to pay and failure to deposit penalties during processing and sends a notice confirming the relief.
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The transition is not finished:
AEP fully replaces First Time Abate for returns with original due dates on or after January 1, 2027. Until then, some eligible taxpayers will still receive a penalty notice, and First Time Abate can still be requested through the old process in those cases.
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Reasonable cause:
Where the clean history is not there, the IRS will consider relief if the failure happened despite ordinary business care and prudence. Serious illness, a death in the family, records destroyed in a disaster. This one needs documentation, not just a narrative.
California’s one-time penalty abatement is separate and works differently. It applies to individuals only, covers tax years beginning in 2022 or later, requires no excuse at all, and can be used exactly once in a lifetime. Estates, trusts and business entities are excluded.
Because the California relief is genuinely once-only, it is worth some thought before spending it. If several years are being caught up at once, apply it to the year carrying the largest penalty rather than the first one processed.
When the IRS Notice Is Likely to Arrive
If you file now, expect a notice a few weeks after the return is processed. It will either show a balance due with the tax, penalties and interest to date, or it will confirm that automatic relief has already been applied. Read which one you have before paying anything.
If penalties were assessed and you believe you qualify for relief, that notice is the moment to ask, not before.
If you do not file at all, the sequence is slower and worse. Reminder notices come first. Eventually the IRS may prepare a substitute return on your behalf using the income reported to it, with no deductions, no credits and no favorable filing status. The assessment that follows is almost always larger than what a real return would have produced, and unwinding it takes far longer than filing would have.
How Alumbra CPA Helps After a Missed Tax Extension Deadline
The order matters more than the speed. File the return first, even if you cannot pay, because that stops the 5% penalty while the 0.5% one continues at a tenth of the cost.
Pay what you can to shrink the base that penalties and daily interest are both calculated on. Deal with entity returns ahead of personal ones, since per-owner penalties climb regardless of the balance owed.
Alumbra CPA helps individuals and business owners across San Diego County resolve late filings, respond to notices and request penalty relief. You can read more about our IRS, state and local tax representation services, or get in touch to find out what an outstanding return is currently costing you.
