1065 and 1120-S Tax Returns: September 15 Deadline - What Your Business Needs to Know

If your business is taxed as a Partnership or S Corporation, pay attention: September 15, 2026 is an important tax deadline for many businesses that properly requested an extension.

For calendar-year businesses that timely requested a six-month extension, September 15, 2026 is the extended deadline to file Form 1065 and Form 1120-S.

In this article, we'll explain who needs to file these forms, the difference between Form 1065 and Form 1120-S, how Schedule K-1 works, the risks of missing the deadline, and what your business should do now.

What Are Forms 1065 and 1120-S?

Both forms are business tax returns used in the United States, but they apply to different business tax structures.

Form 1065

Form 1065 – U.S. Return of Partnership Income is generally used by Partnerships to report business income, expenses, profits, losses, and other tax information to the IRS.

A Partnership generally does not pay federal income tax at the entity level on its ordinary business income. Instead, taxable items generally pass through to the partners.

Form 1120-S

Form 1120-S – U.S. Income Tax Return for an S Corporation is used by businesses that have made a valid election to be taxed as an S Corporation.

Similar to Partnerships, certain income, deductions, credits, and other tax items are generally passed through to shareholders through Schedule K-1.

Why Is the Deadline September 15?

For calendar-year businesses that properly requested a six-month extension, the IRS establishes September 15, 2026 as the extended deadline for Forms 1065 and 1120-S.

The original deadline for these businesses was in March 2026. Because Form 7004 generally provides a six-month extension for these returns, the deadline was extended to September.

In summary:

Form 1065 + valid extension → September 15, 2026

Form 1120-S + valid extension → September 15, 2026

The applicable Schedule K-1s must also be provided to the respective partners or shareholders within the applicable deadline.

Who Needs to File Form 1065?

Form 1065 is generally used by Partnerships.

This may include businesses structured as:

• Multi-Member LLCs taxed as Partnerships;

• General Partnerships;

• Limited Partnerships;

• Other entities treated as Partnerships for federal tax purposes.

  
  

For example, an LLC with two or more owners is generally treated as a Partnership for federal tax purposes unless it makes a different tax election.

It's important to verify your company's tax classification before determining which form it needs to file.

Who Needs to File Form 1120-S?

Form 1120-S is used by a business that has valid S Corporation tax status.

This can include an LLC or Corporation that has made a valid election to be taxed as an S Corporation and meets the applicable requirements.

The S Corporation must file Form 1120-S and provide each shareholder with the appropriate Schedule K-1.

What Is Schedule K-1?

Schedule K-1 is one of the most important documents associated with Forms 1065 and 1120-S.

It reports each partner's or shareholder's share of the company's tax items.

A K-1 may include information related to:

• Income;

• Deductions;

• Credits;

• Losses;

• Distributions;

• Other tax items.

  
  

This information is then used by the owner when preparing their individual tax return.

That's why a delay in the business tax return can also affect the owners' personal tax returns.

Does an Extension Mean You Don't Have to Pay Taxes?

No.

This is one of the most important points to understand.

An extension to file a Tax Return does not automatically mean an extension to pay taxes.

The extension generally provides additional time to file the return, but it does not eliminate or necessarily postpone an existing tax payment obligation.

That's why it's important to understand:

Extension to File ≠ Extension to Pay

Even when a business has a valid extension, any taxes potentially due should be evaluated and paid according to the applicable rules.

What Happens If You Miss the Deadline?

Failing to file the return by the applicable deadline can result in penalties and interest, depending on the circumstances.

For Partnerships and S Corporations, penalties can be calculated based on factors such as the number of partners or shareholders and how long the return is late.

A late business return can also create additional problems for owners, particularly when Schedule K-1 information is needed to prepare their individual tax returns.

That's why it's not a good idea to wait until the final days to organize your documentation.

What Documents Should Your Business Organize?

Before preparing Form 1065 or Form 1120-S, your business should gather information such as:

• Total revenue;

• Business expenses;

• Bank statements;

• Bookkeeping records;

• Payroll records;

• Contractor payments;

• Asset purchases;

• Loans;

• Owner distributions;

• Partner or shareholder information;

• Investment documentation;

• Information needed to prepare Schedule K-1s.

  
  

The more organized your Bookkeeping is, the more efficient the Tax Return preparation process will be.

1065 vs. 1120-S: What's the Difference?

Although both are pass-through structures, their tax and compliance rules are different.

For this reason, you should not choose between a Partnership and an S Corporation based solely on the filing deadline or one particular tax difference.

Does My LLC Need to File Form 1065 or 1120-S?

It depends on the LLC's tax classification.

For example:

Multi-Member LLC

A multi-member LLC is generally treated as a Partnership for federal tax purposes unless it makes a different election.

In that case, the business generally files Form 1065.

LLC Taxed as an S Corporation

If the LLC has made a valid election to be taxed as an S Corporation and meets the applicable requirements, it will generally file Form 1120-S.

That's why it's important to verify your company's tax classification before preparing the return.

What Should You Do Before September 15?

If your business is subject to this deadline, we recommend:

✅ Confirm that your extension was properly filed;

✅ Verify whether your business needs to file Form 1065 or Form 1120-S;

✅ Complete your Bookkeeping;

✅ Reconcile all business bank accounts;

✅ Organize income and expenses;

✅ Review Payroll records;

✅ Verify contractor payments;

✅ Organize partner/shareholder documentation;

✅ Prepare the Schedule K-1s;

✅ Review potential taxes due;

✅ File the return by the applicable deadline.

Don't wait until the final week to get everything organized.

September 15 Is Also an Estimated Tax Payment Deadline

In addition to extended Forms 1065 and 1120-S, September 15 is also an important date for certain 2026 Estimated Tax Payments.

The IRS tax calendar identifies September 15 as the due date for the third estimated tax payment of 2026 for certain taxpayers, including individuals and corporations.

This means that September 15 may involve more than one tax obligation, depending on the taxpayer's situation.

How AES Accounting Can Help

AES Accounting, located in Orlando, Florida, helps business owners and investors prepare and organize their U.S. tax obligations.

Our services include:

• Form 1065;

• Form 1120-S;

• Schedule K-1;

• Tax Returns;

• Tax Planning;

• Bookkeeping;

• Payroll;

• Business Compliance;

• LLC Formation;

• S Corporation Planning.

  
  

Our team can help your business organize its documentation, identify applicable filing requirements, and prepare your Tax Return before the applicable deadlines.

Conclusion

If your business is a Partnership or S Corporation and you properly requested an extension for the 2025 calendar tax year, September 15, 2026 is a deadline you cannot ignore.

The extended deadline applies, among others, to Form 1065 for Partnerships and Form 1120-S for S Corporations, along with the applicable Schedule K-1s.

And remember: an extension to file does not necessarily mean an extension to pay taxes.

If you haven't organized your business Tax Return yet, now is the time to act.

AES Accounting. Specialists in Tax Returns, Form 1065, Form 1120-S, Schedule K-1, Bookkeeping, Tax Planning, Payroll, and Business Compliance for entrepreneurs and investors throughout the United States.