Key September 2026 Tax Deadlines for Businesses

September brings several important tax deadlines for businesses, including extended returns for S corporations and partnerships, corporate estimated tax payments, and employer payroll tax deposits. Whether you are operating right here in California or across the country, reviewing these key due dates will help keep your business filings and payments on track.

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September 15: S Corporation Extended Returns

If you requested an automatic six-month extension earlier this year, September 15 is your deadline to file a 2025 calendar year income tax return (Form 1120-S) and pay any remaining tax due. You must also provide each shareholder with a copy of their Schedule K-1 (Form 1120-S) or a substitute Schedule K-1. If applicable to your entity, provide a Schedule K-3 (Form 1120-S) or a substitute Schedule K-3 as well.

September 15: Corporate Estimated Taxes

For calendar year corporations, this date marks the deadline to deposit the third installment of your estimated income tax for 2026.

September 15: Partnership Extended Returns

If your partnership timely requested an automatic six-month extension, you need to file your 2025 calendar year return (Form 1065) by September 15. Just like S corporations, you are required to provide each partner with a copy of their Schedule K-1 (Form 1065) or a substitute, along with a Schedule K-3 (Form 1065) or substitute if applicable.

September 15: Payroll and Withholding Deposits

If you are an employer subject to the monthly deposit rules, September 15 is the due date to deposit your Social Security, Medicare, and withheld income tax for August 2026. If the monthly deposit rule applies to your business, this is also the deadline for your nonpayroll withholding deposit for August 2026.

September 30: Fiduciary Returns for Estates and Trusts

Fiduciaries of estates and trusts who were granted a 5½-month extension must file a 2025 calendar year return (Form 1041). If applicable, remember to provide each beneficiary with a copy of their Schedule K-1 (Form 1041) or a substitute.

Weekends and Legal Holidays

If a due date happens to fall on a Saturday, Sunday, or legal holiday, the IRS automatically extends that due date until the next business day that is not itself a legal holiday.

Disaster Area Extensions

Keep in mind that when a geographical area is formally designated as a disaster area, tax due dates will be extended. To verify if your location qualifies and to find your specific filing extension dates, visit the following resources:

Stay Ahead of Your Fall Deadlines

Missing a due date can result in unnecessary penalties and stress for your business. If you need assistance managing these extended returns, corporate estimated payments, or payroll deposits, schedule a consultation with the team at Christiansen Accounting today.

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