Filed a Tax Extension? Your S-Corp or Partnership Return Is Due September 15

If your business filed for an extension back in March, the clock runs out on September 15 for S-corporations filing Form 1120-S and partnerships filing Form 1065. Unlike an individual extension, there is no second one available after this date.

Why the September 15 Deadline Matters More Than It Seems

Your partners and shareholders need their K-1s to file their own personal returns, which are due October 15. The later you file, the less time everyone else has to work with.

What It Costs to Miss the S-Corp or Partnership Deadline

Late-filing penalties for S-corps and partnerships are charged per partner or shareholder, per month. A business with four owners racks up penalties four times faster than a solo filer, so the cost adds up quickly for multi-owner businesses.

Your Last-Minute Filing Checklist

• Reconcile your books through December 31, 2025
• Confirm K-1 allocations match your partnership or operating agreement
• Double-check basis calculations, especially for real estate partnerships with debt
• Verify any Illinois PTET election was made correctly
• File electronically so you get same-day confirmation

That first item is usually the one that holds everything else up. If your books are not fully reconciled, the rest of the checklist is guesswork rather than a filing you can trust.

Frequently Asked Questions About the September 15 Deadline

I’m a Partner, What Do I Do With My K-1?

Send it to your tax preparer right away, since you’ll need it for your personal return, due October 15.

What If We’re Not Going to Make the Deadline?

There is no extension beyond this one, so the goal becomes filing as soon as possible to limit how many months of penalties add up.

At JIL & Associates, we specialize in S-corp and partnership tax returns for businesses across the Chicago area, and as an Advanced QuickBooks ProAdvisor, we can also get your books reconciled and caught up before you file. We’ll help you file accurately and get your K-1s out on time. Call (630) 828-3648 or schedule a free consultation.