If you filed a tax extension back in April, the clock runs out this week. Your 2025 return is due Thursday, October 15, 2026, and there is no second extension.
We talk to a lot of business owners who filed an extension and then let it slip to the bottom of the pile. That's understandable. Running a business is a full-time job before you add tax paperwork to it. But missing this date is one of the more expensive mistakes you can make, so here's what you need to know and a checklist to get it done.
Who has to file by October 15?
This deadline applies if you requested an extension for any of these:
- Individual returns (Form 1040). That includes sole proprietors and single-member LLCs reporting on Schedule C, plus owners who receive K-1s from a partnership or S corporation.
- Calendar-year C corporations (Form 1120).
- California returns. The Franchise Tax Board's automatic extension for individuals also runs to October 15.
Partnerships (Form 1065) and S corporations (Form 1120-S) were due on September 15, so if those business returns haven't been filed yet, they're already late. Get them done right away so you can file your personal return on time.
An extension to file is not an extension to pay
This is the part that surprises people. Your extension gave you more time to finish the paperwork, but any tax you owed was still due on April 15. If you didn't pay it all then, interest and a failure-to-pay penalty have been building since April.
The good news: if you're owed a refund, there's no penalty for filing late, because penalties are based on unpaid tax. You're just waiting longer for your money.
Why filing late costs more than paying late
The IRS treats these two problems very differently:
| Penalty | Rate | Maximum |
|---|---|---|
| Failure to pay | 0.5% of unpaid tax per month | 25% |
| Failure to file | 5% of unpaid tax per month | 25% |
The failure-to-file penalty is ten times larger. When both apply in the same month, the filing penalty is reduced by the payment penalty, so the combined hit is about 5% per month. On a $20,000 balance, that's roughly $1,000 for every month you wait. Interest comes on top of that.
The takeaway: even if you can't pay a dime right now, file the return on time. It stops the larger penalty from starting.
Your last-minute checklist
Gather these before you (or your CPA) sit down to finish the return:
Business income and expenses
- 2025 books closed and reconciled, with a final profit and loss statement and balance sheet
- All 1099s you received (1099-NEC, 1099-MISC, 1099-K from payment processors)
- K-1s from any partnership or S corporation you own a piece of
- Vehicle mileage logs and home office square footage, if you claim them
- Records for any equipment or vehicles you bought during 2025
Payments you already made
- Dates and amounts of all four 2025 estimated tax payments (federal and California)
- Any payment you sent with your April extension
Personal items
- W-2s, 1099-INT, 1099-DIV and 1099-B brokerage statements
- Mortgage interest (Form 1098) and property tax statements
- Form 1095-A if you had Marketplace health insurance
- Retirement contribution records
- A copy of your 2024 return
Self-employed? Don't miss this one. Because you extended, you can generally still make a 2025 SEP-IRA contribution, as long as the money lands in the account by October 15. That can be a meaningful deduction, so it's worth a quick conversation with your CPA before you file.
What if you can't pay what you owe?
File anyway. Then look at your options:
- Pay what you can now. Every dollar paid reduces future penalties and interest.
- Set up a payment plan. Most people who owe can apply online at IRS.gov without calling anyone.
- Talk to your CPA. If you've had a hard year, there may be penalty relief options worth exploring.
After you file
- Confirm the e-file was accepted. A rejected return that never gets fixed counts as unfiled.
- Mark January 15, 2027 on your calendar. That's when your fourth-quarter 2026 estimated payment is due.
- Start 2026 planning now. You have about eleven weeks left in the year to make moves that lower next April's bill. Waiting until spring means most of those options are gone.
Running out of time? We can help.
Accounting Fresh CPA helps small business owners get caught up, get filed and get a plan in place so next year isn't a scramble.
Schedule a CallOr call us at 760-230-9779
This article is for general information only and is not tax or legal advice for your specific situation. Tax rules change and individual circumstances vary, so please consult a qualified professional before acting.