September 15 and October 15 Tax Deadlines: What Businesses and Individuals Need to Know
Fall brings two important federal tax deadlines. Whether you own a business, make quarterly estimated tax payments, or extended your personal return, now is the time to make sure your information is complete and ready.
Here is what you need to know about the September 15 and October 15, 2026 deadlines.
September 15, 2026: Business Returns and Estimated Tax Payments
Extended Partnership Returns
Calendar-year partnerships that requested an extension must file their 2025 federal return, Form 1065, by September 15.
The partnership must also provide each partner with a final Schedule K-1. Partners need this information to complete their personal tax returns, which may be due October 15 if extended.
Extended S Corporation Returns
Calendar-year S corporations that requested an extension must file their 2025 federal return, Form 1120-S, by September 15.
Each shareholder must also receive a final Schedule K-1. Because S corporation income generally passes through to the shareholders, this information is essential for completing their personal returns.
Third-Quarter Individual Estimated Tax Payments
September 15 is also the due date for the third individual estimated tax payment for 2026.
Estimated payments may apply to:
Sole proprietors
Partners and S corporation shareholders
Independent contractors
Owners of rental properties
Individuals with significant investment income
Anyone who does not have enough federal tax withheld from wages or other income
If your business income has changed significantly during 2026, this is a good time to review your estimated payments. Paying the same amount as last year may not be appropriate if your income, deductions, or business circumstances have changed.
Corporate Estimated Tax Payments
Calendar-year C corporations may also have a third-quarter estimated federal income tax payment due September 15.
October 15, 2026: Personal Returns and C Corporation Returns
Extended Individual Income Tax Returns
Individuals who timely requested an extension must file their 2025 federal income tax return—Form 1040 or Form 1040-SR—by October 15.
This deadline includes personal returns reporting income from:
Wages and retirement distributions
Sole proprietorships and single-member LLCs
Partnerships and S corporations
Rental properties
Investments
Other personal income sources
An extension provides additional time to file the return, but it does not provide additional time to pay the tax. Any 2025 federal income tax owed was generally due in April. Interest and possible penalties may continue to accumulate on unpaid balances.
Even if you cannot pay the entire balance, it is generally better to file the return on time and then address available payment options.
Extended C Corporation Returns
Calendar-year C corporations that timely requested an extension must file their 2025 federal income tax return, Form 1120, by October 15.
Why Business Owners Need to Watch Both Deadlines
Many business owners are affected by both dates.
For example, an S corporation or partnership return may be due September 15, while the owner’s personal return is due October 15. The business return must be completed first so the owner has the Schedule K-1 needed to finish the personal return.
Delays in completing the business books can therefore create delays for multiple returns—not only the company’s return, but also the returns of its owners.
Do Not Wait Until the Final Week
Before filing, business owners may need time to:
Reconcile bank and credit card accounts
Review payroll and contractor payments
Confirm fixed asset purchases and loan balances
Identify missing income or expenses
Resolve bookkeeping discrepancies
Complete year-end adjusting entries
Review shareholder or partner transactions
Gather documents for deductions and tax credits
Submitting complete information early gives your tax professional time to identify missing items, resolve questions, and prepare an accurate return.
Now Is Also a Good Time for Tax Planning
Filing a 2025 return tells us what happened last year. Tax planning helps us make better decisions for 2026.
Business owners should consider reviewing:
Year-to-date profit
Estimated tax payments
Owner compensation
Retirement plan contributions
Major equipment purchases
Cash flow and upcoming tax obligations
Changes in business structure or ownership
A short review before year-end can help prevent surprises and identify planning opportunities while there is still time to act.
Need Help Meeting an Upcoming Deadline?
If your business or personal return is still on extension, contact CB Accounting as soon as possible. We can help identify the information that is still needed, address bookkeeping issues, and determine the best path toward completing your return.
Do not let the September 15 or October 15 deadline sneak up on you. A little preparation now can make the filing process far less stressful.
This article provides general information and should not be considered tax advice for a specific situation. State and local filing requirements may differ, and special federal deadlines may apply to certain taxpayers.How CB Accounting Helps Growing Construction Companies
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Advisory Services Are the Smartest Investment for Growing Businesses
RERReady for more than the numbers?
Most business owners already have an accountant.
What many don’t have is a strategic financial partner helping them increase profits, improve cash flow, and make smarter business decisions.
That’s the difference advisory services make.
Today’s businesses need more than year-end tax filings and financial statements. They need proactive guidance, real-time insights, and a trusted advisor who can help them navigate growth, challenges, and opportunities with confidence.
Traditional Accounting Tells You What Happened. Advisory Helps You Decide What Happens Next.
Compliance services are essential — but they’re only part of the picture.
Bookkeeping records transactions. Tax returns report history. Financial statements summarize the past.
Advisory services take things further by helping business owners understand:
Where their business is headed
What’s impacting profitability
How to improve cash flow
When it’s safe to hire, expand, or invest
How to make more informed financial decisions
Instead of simply delivering reports, advisory-focused firms help clients use their numbers to drive growth.
Business Owners Want More Than Compliance
Many entrepreneurs feel overwhelmed trying to manage growth while keeping finances under control.
They’re asking questions like:
Why are revenues increasing but cash flow still tight?
Which areas of the business are actually profitable?
How can we reduce unnecessary expenses?
Are we prepared for slower economic periods?
What should we focus on to scale successfully?
Without financial guidance, business owners are often making critical decisions based on instinct instead of data.
Advisory services provide the clarity and strategy businesses need to move forward confidently.
The Real Value of Advisory Services
Better Financial Visibility
Business owners gain a clear understanding of where their money is going and what’s driving performance.
Improved Profitability
By analyzing margins, expenses, and operational trends, advisors help identify opportunities to increase profits.
Stronger Cash Flow Management
Cash flow issues are one of the biggest challenges for growing businesses. Advisory services help clients forecast and plan ahead before problems arise.
Smarter Growth Decisions
Whether it’s hiring staff, purchasing equipment, or expanding operations, strategic financial guidance helps reduce risk and improve decision-making.
Ongoing Support — Not Just Once a Year
Clients receive proactive insights throughout the year instead of only hearing from their accountant during tax season.
Why Businesses Are Choosing Advisory-Focused Firms
The accounting industry is changing rapidly.
Business owners no longer want a reactive accountant who only files taxes and closes the books. They want a trusted advisor who understands their goals and actively helps them succeed.
Firms that provide advisory services become:
That creates stronger client relationships and significantly more value for businesses.
Advisory Services Are an Investment — Not an Expense
One of the biggest misconceptions about advisory services is that they are an added cost.
In reality, the right financial guidance can help businesses:
Improve profitability
Avoid costly mistakes
Identify growth opportunities
Increase operational efficiency
Make better long-term decisions
The return on strategic financial advice often far outweighs the investment.
The Future of Accounting Is Advisory
Technology and automation are changing traditional accounting services. As bookkeeping and compliance become more automated, businesses are placing greater value on insight, strategy, and personalized guidance.
That’s why advisory services are quickly becoming one of the most valuable services an accounting firm can offer.
Businesses don’t just need someone to prepare financial reports.
They need someone who can help them understand the story behind the numbers — and use that information to build a stronger, more profitable future.
Ready to Take a More Strategic Approach to Your Business Finances?
At CB Accounting LLC, we help businesses move beyond basic compliance by providing proactive financial guidance designed to support growth, profitability, and long-term success.
If you’re looking for more than just bookkeeping and tax preparation, advisory services may be the next step for your business.