Generic tax tips are everywhere. “Track your expenses.” “Save your receipts.” “Work with a professional.” These aren’t wrong — they’re just not useful to anyone who’s been running a business for more than a year.
What’s actually useful is knowing the specific places where small business owners consistently leave money behind or create problems they didn’t need to have.
Here are the ones we see most often.
Getting the Business Structure Wrong — and Keeping It Wrong
The most expensive tax mistake most small business owners make isn’t a deduction they missed. It’s operating in the wrong business structure for their income level.
A sole proprietor pays self-employment tax — 15.3 percent — on every dollar of net business profit. On $100,000 of net profit that’s $14,130 in self-employment tax before a dollar of federal income tax is calculated.
An S-Corp splits that income between a salary — which is subject to payroll taxes — and distributions — which are not. On the same $100,000 with a $60,000 reasonable salary, self-employment tax applies only to the $60,000. The remaining $40,000 in distributions avoids it entirely. That’s roughly $6,000 in annual savings on one structural decision.
The S-Corp election generally makes sense once net annual profit is consistently high enough that the payroll tax saved outweighs the added costs. The election has a deadline — March 15 for the current tax year — and the setup involves payroll and compliance requirements that need to be in place. But for a profitable business that’s been operating as a sole proprietor for years, the accumulated cost of not making this decision is significant.
If you’ve never had this conversation with a tax professional, you’ve probably been overpaying.
Missing the Retirement Contribution Window
Most small business owners contribute far less to retirement than they’re eligible to — and it shows up directly in their tax bills.
A solo 401k allows combined contributions of up to $72,000 in 2026 for owners under 50. Every dollar contributed reduces taxable income by the same amount. A business owner in the 22 percent federal bracket who contributes an additional $20,000 saves $4,400 in federal income tax.
The employee contribution portion of a solo 401k must be made by December 31. The employer contribution portion can be made up to the tax filing deadline. But the plan itself has to be established before December 31 of the year you want to make contributions. If you’ve never set one up, that’s a year-end deadline worth knowing.
SEP IRAs are simpler to establish and allow contributions up to 25 percent of compensation — roughly 20 percent of net self-employment earnings for sole proprietors, made any time before the tax filing deadline.
The missed retirement contribution is one of the most common and most correctable tax mistakes we see. The opportunity is real. It just requires someone to flag it before the deadline passes.
Most of these mistakes are cheaper to prevent than to fix. Have us look over how your business handles them before year end.
Confusing Tax Preparation With Tax Planning
Paying someone to file your return is not the same as having someone actively work to reduce your tax bill.
Tax preparation is recording what happened. Tax planning is making decisions before they happen — in ways that reduce what you’ll owe when the return is filed.
By the time you’re sitting down with a preparer in March, most of the decisions that determined your tax bill are already locked in. The income has been earned. The expenses have been paid. The retirement contribution window has mostly closed.
The business owners who consistently pay less in taxes aren’t smarter or luckier. They’re having planning conversations throughout the year — in the summer about where things are tracking, in the fall about year-end moves, in January about what the next year should look like.
If your only tax conversation of the year happens at filing time, you’re getting compliance. You may not be getting optimization.
Not Tracking Mileage
The 2026 standard mileage rate for business driving is a flat 72.5 cents per mile for the full year. At 72.5 cents, 15,000 business miles is a $10,875 deduction — but only with documentation.
The IRS requires a contemporaneous mileage log — meaning you track trips as they happen, not at year end from memory. Date, destination, business purpose, miles driven. That’s all it takes. There are apps that automate most of this.
Claiming mileage without a log is audit risk. Not claiming mileage you actually drove is leaving real money behind. The solution is a two-minute habit applied consistently.
Underpaying Quarterly Estimated Taxes
Self-employed business owners and small business owners don’t have taxes withheld automatically. They’re responsible for making quarterly estimated payments — and the IRS charges a penalty when those payments are insufficient.
The penalty isn’t catastrophic but it’s real and it compounds over time. More importantly, the surprise tax bill that comes from not paying throughout the year causes real cash flow problems for businesses that haven’t been setting money aside.
The safe harbor rule eliminates the penalty: pay 100 percent of your prior year tax liability in four equal quarterly installments and you avoid the underpayment penalty regardless of what you owe. If your prior year adjusted gross income exceeded $150,000, the safe harbor is 110 percent of prior year liability.
First-year business owners are the most likely to miss this entirely. If you started a business this year and haven’t made any estimated payments, a conversation with a tax professional about Q4 is worth having now.
Mixing Personal and Business Finances
This one isn’t just a tax problem — it’s a bookkeeping problem that creates tax problems.
When business and personal transactions run through the same accounts, categorizing expenses correctly becomes a reconstruction project rather than a normal process. Deductions get missed. Some things get miscategorized. Your books at year end are less accurate than they should be.
The fix is simple and one-time: open a dedicated business checking account and a dedicated business credit card. Every business transaction runs through those accounts. Every personal transaction runs through personal accounts. The separation eliminates the problem at the source.
None of these are once-a-year conversations. With our small business clients they come up throughout the year, while there's still time to do something about them.