September 15 is coming up quickly, and for many business owners it is an important tax deadline that can be easy to forget about once the spring tax season is behind us.
If you own an S corporation or are part of a partnership and filed an extension earlier this year, September 15 is generally the deadline for getting that business return filed. For calendar-year S corporations, that means Form 1120-S, and for partnerships, it means Form 1065.
When an extension is filed in the spring, September can feel like it is a long way off. Then summer gets busy, the months go by, and suddenly the extended deadline is right around the corner. If your return still needs to be completed, now is the time to make sure your bookkeeping is caught up and your tax preparer has everything needed to finish the return.
This is especially important for S corporations and partnerships because the business return may also affect the owners’ personal tax returns. These businesses generally issue a Schedule K-1 to each shareholder or partner. That information is then used when preparing the individual’s tax return. When the business return is delayed, it can hold up the personal return as well.
September 15 is also the third estimated tax payment deadline for 2026. This is something many small business owners, independent contractors and self-employed individuals need to pay attention to because taxes aren’t always automatically withheld from the income they receive.
Now is also a good time to look at how your business has actually performed this year. The income you expected at the beginning of 2026 may be very different from where you are today. Maybe business has been better than expected, you picked up a few large clients, or you added another source of income. On the other hand, business may have slowed down or you may have had some significant expenses that weren’t anticipated earlier in the year.
Those changes can affect how much you should be paying in estimated taxes. Rather than automatically assuming the amount you calculated months ago is still appropriate, it can be worth taking another look at the numbers.
Another thing I see people misunderstand is what a tax extension actually does. An extension gives you additional time to file your return, but it generally does not give you additional time to pay taxes that were already due. If you filed an extension and haven’t looked at your tax situation since then, you may want to find out where things stand before the September deadline arrives.
If you owe more than you expected, don’t ignore the return because you are worried about paying the balance. Filing the required return and dealing with what is owed is generally a much better approach than allowing the problem to grow. Depending on the circumstances, there may also be options available when someone cannot pay a tax balance in full.
Even if September 15 isn’t a filing deadline for your particular business, September is a good time to take a look at your taxes. We are far enough into the year to have a pretty good idea of how the business is performing, but there are still several months left before the end of the year.
That gives us time to look at income and expenses, estimated payments, payroll, retirement contributions, upcoming purchases and other financial decisions before December 31. There may be things we can address now that we won’t be able to change once the year is over.
This is one of the reasons I encourage business owners not to think about taxes only during tax season. By the time we are preparing a return next year, we are looking backward at decisions that have already been made. When we review things before the end of the year, we have an opportunity to plan ahead.
If you have a September 15 deadline, don’t wait until the last few days to start pulling everything together. Make sure your bookkeeping is current, check whether your tax preparer is waiting for anything from you, and take care of any unanswered questions now.
And if you’re not sure whether the September 15 deadline applies to you, that’s something we can help you determine.
At TaxPointe, we work with individuals and business owners throughout the year to help them stay on top of their tax obligations and avoid unnecessary surprises. If you have an extended return that still needs to be filed, need help with your estimated taxes, or simply want to review where you stand heading into the last few months of 2026, contact TaxPointe and we’ll be happy to help.

