September 15 Is a Bigger Tax Deadline Than Most Business Owners Realize
September 15 doesn’t get the same attention as April 15, but for a lot of business owners, it’s a pretty important tax day.
It’s the due date for the third 2026 estimated tax payment for individuals who need to make estimated payments. It’s also the extended filing deadline for calendar-year partnerships and S corporations.
Need help figuring out what still needs to happen before the deadline? Schedule a consultation with Dunham Bookkeeping Services. A few days before the deadline is much better than discovering a problem a few days after it.
First, check your estimated tax payment
Estimated taxes commonly apply to business owners whose income doesn’t have enough tax withheld throughout the year.
That can include sole proprietors, partners, S corporation shareholders, and people with other significant income outside of a regular paycheck.
The September payment is the third estimated tax installment for 2026.
But don’t just find an old payment voucher and assume the number is still right.
A lot can change between January and September. Your business may be having a much better year than expected. Revenue may have dropped. You may have started taking payroll from an S corporation, changed jobs, sold an investment, picked up a new income stream, or had some other major financial change.
Your estimated payments should reflect what’s actually happening now, not what everyone thought would happen months ago.
Make sure the earlier payments actually happened
This sounds obvious.
I promise it is worth checking anyway.
Look at the payments you intended to make in April and June and verify that they actually cleared your bank account and were applied correctly.
A spreadsheet saying “paid” is not the same thing as the IRS receiving the payment.
Finding a missing payment now gives you a chance to address it instead of discovering it while preparing your tax return.
Extended partnership and S corporation returns are also due
September 15 is also the filing deadline for calendar-year partnerships and S corporations that received a valid six-month extension.
That means Forms 1065 and 1120-S need to be completed and filed, along with the related Schedule K-1s.
An extension gave you more time to file. It did not make the return disappear into the tax-season void forever. Unfortunately.
Waiting until the last minute can create another problem too: owners need those K-1s for their personal tax returns.
A late business return can create a domino effect that delays everyone involved.
An extension doesn’t automatically mean everything is fine
Having an extension on file is useful, but it shouldn’t become a reason to stop paying attention to the return.
By September, you should know whether the bookkeeping is complete, accounts are reconciled, major questions have been answered, and your tax preparer has the documents needed to finish the return.
This is especially important when the business has had changes during the year such as a new owner, ownership transfer, large asset purchase, loan, payroll change, or unusual transaction.
Those are not great surprises to introduce on September 14.
Use the deadline as a midyear tax checkup
Even business owners who don’t have an extended entity return can use September 15 as a reminder to look at the bigger tax picture.
Take a look at year-to-date profit. Review your payroll withholding. Confirm estimated payments. Think about any significant changes that happened this year.
There is still time left in 2026 to make thoughtful decisions.
That’s much more useful than waiting until tax preparation starts and finding out what you wish you had done six months earlier.
Need help with estimated taxes, business tax preparation, or getting your bookkeeping ready for tax season? Schedule a consultation with Dunham Bookkeeping Services.
