Tuesday, September 15, 2026, is the federal extended filing deadline for calendar-year partnerships (Form 1065) and S corporations (Form 1120-S) - but only if your entity obtained a timely, valid extension. This date applies to 2025 tax returns and assumes the IRS e-file submission was accepted.
Here’s what Orange County business owners need to know, what’s due, and the steps to take before the deadline.
Which Returns Are Due September 15?
- Partnerships (including most multi-member LLCs taxed as partnerships) filing Form 1065
- S corporations filing Form 1120-S
The original deadline was March 16, 2026 (because March 15 fell on a Sunday). A valid Form 7004 extension gives these entities six additional months, pushing the due date to September 15.
Quick test: Is your business a calendar-year partnership or S corporation with a valid extension? If yes, September 15 applies to you.
Not sure which applies? Many LLCs are taxed as partnerships, but that’s not automatic. Some elect S corporation status. Learn more about your entity’s tax classification before assuming a deadline.
What Must Be Done by September 15?
This is not a second extension request date. Form 7004 had to be filed by the March regular deadline. By September 15, you need:
- Complete return: All income, deductions, gains, losses, and ownership details finalized
- Schedules K-1/K-3 prepared: Provided to every partner or shareholder who held an interest during the year
- E-file accepted: “Submitted” isn’t “accepted.” Retain the IRS acceptance notice and resolve any rejections immediately.
Schedule K-1 matters because owners use it to complete their personal, trust, estate, or corporate returns. Late or corrected K-1s delay owner filings and can trigger amended returns.
A Filing Extension Does Not Extend Payment
Form 7004 postpones the filing deadline, not tax payments. Any balance due was generally payable by the original March deadline. Interest runs from that original date, even with a valid extension.
The pass-through structure complicates this:
- Partnerships generally don’t pay federal income tax at the entity level, but may owe other federal or state liabilities
- S corporations can owe entity-level tax on built-in gains and passive income
- Owners have separate estimated-tax obligations unaffected by Form 7004
Action step: Review unpaid balances and payment history with your business tax professional at Mraz Tax Solutions LLC promptly.
What If You Miss the Deadline?
Late-filing penalties multiply by both the number of owners and the number of months late:
| Form | Penalty per month | Cap |
|---|---|---|
| Form 1065 (per partner) | $255 | 12 months |
| Form 1120-S (per shareholder) | $255 | 12 months |
| Incorrect/late K-1 | $340 per schedule | Various caps |
These are 2026 inflation-adjusted figures. Reasonable-cause relief is possible, but it’s fact-specific and determined by the IRS - never guaranteed.
Preparing for S Corps vs. Partnerships
S corporations:
- Reconcile shareholder compensation and payroll
- Verify health-insurance reporting
- Document distributions and shareholder loans
Partnerships:
- Confirm partner percentages and special allocations
- Review liabilities, capital accounts, and guaranteed payments
- Document transfers of interests
For both, our S corporation and partnership tax services can help you reconcile accounts, verify ownership details, and prepare K-1s that won’t create problems downstream.
Before You File: Final Checklist
- Confirm a valid Form 7004 was filed (retain e-file acceptance)
- Reconcile year-end bank, credit-card, and loan accounts
- Resolve uncategorized transactions
- Verify income against Forms 1099
- Gather fixed-asset and depreciation information
- Identify state pass-through entity tax elections and payments
- Review foreign accounts and cross-border transactions (K-2/K-3 applicability)
- Review every K-1: names, TINs, ownership details, allocations
- Transmit early to correct rejected submissions
- Deliver final K-1s securely to owners
Caveats That Could Change Your Deadline
- Fiscal-year entities: Different due dates apply. This deadline is for calendar-year taxpayers.
- No valid extension: Your return is already late. Filing now doesn’t retroactively create an extension.
- Disaster relief: Check current IRS disaster notices if you’re in a covered area.
- State filings: Federal extensions don’t automatically cover California and other state returns. Verify separately.
This article addresses the general federal deadline and is not tax advice. Fiscal-year returns, state filings, disaster relief, and entity-specific facts can change the applicable due date and requirements.
Need help before September 15? Contact Mraz Tax Solutions LLC to review your filing status, K-1 preparation, and compliance.