Extended Your 2025 Return? What to Finish Before October 15, 2026

If you extended your 2025 individual return, it's due October 15, 2026, for both the IRS and California. The extension gave you more time to file, not more time to pay, so any balance has been building interest since April. Filing on time this month stops the larger of the two penalties from ever starting.

What the extension covered, and what it didn't

The automatic extension for individuals runs six months from the April due date, which puts the 2025 deadline at October 15, 2026 (IRC §6081(a); IRS, Get an extension to file your tax return). The IRS says it plainly: the extension is only for filing your return. California says the same thing: its automatic six-month extension to October 15, 2026, "is not an extension to pay" (FTB, Extension to file).

Calendar-year C corporations that filed Form 7004 have the same October 15 date for their 2025 Form 1120. For a calendar year beginning before January 1, 2026, the corporate extension is five months instead of six (IRC §6081(b)), which lands on the same day.

The two penalties, and why only one is still in your control

There are two separate additions to tax under IRC §6651, and they work very differently.

• Failure to file is 5% of the unpaid tax for each month or part of a month the return is late, up to 25% (IRC §6651(a)(1)). It's measured from the extended due date. If you file by October 15, it never starts.
• Failure to pay is 0.5% a month, also capped at 25% (IRC §6651(a)(2)). It's measured from the original April 15 date, because you got an extension to file, not to pay.

Interest is separate from both. It runs from the original due date, determined without regard to any extension (IRC §6601(b)(1)), and it isn't waived for reasonable cause.

So the failure-to-file penalty is the one you can still avoid entirely, and it's ten times the monthly rate of the other one. If the return isn't quite finished, filing a complete and accurate return on time and amending later for a late document is usually better than filing late.

The rule most people don't know about

The regulations give individuals on an automatic extension a useful safe harbor. Reasonable cause for the failure-to-pay penalty is presumed for the extension period if:

1. the tax you paid by the April due date (withholding, estimated payments and any payment with the extension) left an unpaid balance no greater than 10% of the tax shown on the return, and
2. you pay the balance with the return (Treas. Reg. §301.6651-1(c)(3)).

Corporations have a parallel rule: the presumption applies if at least 90% of the Form 1120 tax was paid by the regular due date or shown on Form 7004, and the balance is paid by the extended due date (Treas. Reg. §301.6651-1(c)(4)).

Miss the October deadline and the presumption is gone, along with the failure-to-file exposure.

A worked example

This is an example with round numbers, not anyone's return. Say your 2025 return will show $30,000 of total tax, and you paid $28,000 by April 15, 2026, through withholding, estimates and the payment you sent with your extension. The unpaid $2,000 is under 10% of the tax shown. If you file by October 15 and pay the $2,000 with the return, reasonable cause is presumed for the failure-to-pay penalty during the extension period. You still owe interest on the $2,000 from April 15.

Now say you file on November 20 instead. The extension no longer protects you, the failure-to-file penalty applies for two months (the late period runs into a second month), and the reasonable cause presumption doesn't apply, because it requires filing within the extension.

What I'd do this week

• Collect the late documents. The usual holdups are Schedules K-1 from partnerships and S corporations, corrected 1099-B and 1099-DIV forms from brokers, and basis records for anything sold. If a K-1 truly won't arrive, talk to your preparer about filing on the best information available and amending.
• Check what you paid by April 15, 2026. Add up withholding, estimated payments and the extension payment, and compare it with the tax on the return. That tells you whether the 10% safe harbor is in reach.
• Pay the balance with the return, federal and California. Interest is still running, so waiting doesn't help.
• File California the same day. The FTB deadline is also October 15, 2026.
• Set your fourth-quarter 2026 estimate while the 2025 numbers are in front of you. The last 2026 installment is due January 15, 2027, and your 2025 tax is the starting point for the prior-year safe harbor. Run it in the quarterly estimated tax calculator.

If you extended because the return is complicated, a K-1 from a new investment, a business sale or a move into or out of California, I'm glad to look at it before the deadline. Send an inquiry, or see how I handle tax preparation and planning.

General information, not advice for your situation. The rules and dates here are for 2025 returns filed in 2026, as checked on the date above.