2026 Small Business Tax Planning: The October-to-December Action Checklist

The 90-day window from October through December represents the highest-leverage tax planning period of the year for small business owners. By October, the year's actual financial performance is largely visible, allowing data-driven decisions that combine current-year impact with strategic positioning for the following year. The taxpayers who consistently produce the best after-tax outcomes treat year-end as a structured planning event — not as last-minute scrambling.

October: Foundation and Projection

October is the time to establish accurate year-end projections.

Year-End Tax Projection

1. Compile year-to-date P&L through September 30.

2. Project Q4 income and expenses based on current trends and known events.

3. Calculate projected federal and state tax liability.

4. Compare to year-to-date tax payments (withholding + estimated payments).

5. Identify whether the business is over- or under-paying for the year.

Marginal Rate Analysis

Calculate the projected marginal tax rate on additional income or deductions. For 2026 married couples filing jointly, taxable-income bracket ceilings are:

• 22% bracket through $211,400.

• 24% bracket through $403,550.

• 32% bracket through $512,450.

• 35% bracket through $768,700.

• 37% above $768,700.

Plus state tax, SE tax, NIIT, and Additional Medicare tax for high earners.

QBI Deduction Threshold Review

For pass-through business owners, project taxable income relative to the 2026 §199A thresholds and phase-in ranges: $403,500 to $553,500 for married filing jointly, $201,775 to $276,775 for married filing separately, and $201,750 to $276,750 for all other returns. Section 199A is now permanent; income management can still affect the W-2 wage, UBIA, and specified-service-business limitations.

November: Action and Execution

Retirement Plan Optimization

• Solo 401(k) employee deferral: The 2026 elective-deferral limit is $24,500. If the plan permits, the general age-50 catch-up is $8,000 and the higher catch-up for ages 60 through 63 is $11,250.

• SEP-IRA / Solo 401(k) employer contribution: For employees and corporate owner-employees, the employer contribution is generally limited to 25% of eligible compensation, subject to plan and annual limits. A self-employed owner uses the special reduced-rate calculation, generally up to 20% of adjusted net self-employment earnings after the applicable adjustments. Confirm the amount and payment deadline for the entity and plan.

• Defined benefit plan: A properly designed plan may permit a substantial deductible contribution, but the amount depends on actuarial design, compensation, age, employee coverage, and funding rules. Federal law may permit retroactive adoption by the employer's return due date, including extensions; elective-deferral, notice, amendment, and funding deadlines are separate and plan-specific, so confirm timing with the plan administrator before year-end.

• HSA contributions: For eligible individuals in 2026, the limit is $4,400 for self-only HDHP coverage or $8,750 for family coverage, plus a $1,000 catch-up at age 55 or older.

Equipment and Capital Expenditures

Decide on year-end capital purchases:

• Section 179 elective expensing up to $2,560,000 for 2026, subject to the taxable-income limitation and a phaseout beginning when eligible property placed in service exceeds $4,090,000.

• Bonus depreciation at 100% for eligible property acquired after January 19, 2025, subject to the statutory transition rules and placed-in-service requirements.

• Heavy SUV §179 sub-limit: $32,000 for 2026.

• De minimis safe harbor: $2,500/$5,000 per item for immediate expensing of small purchases.

• Placement in service deadline: December 31 — equipment must be in service, not just purchased.

S-Corp Salary True-Up

For S-corp owners:

• Verify W-2 wages are tracking toward reasonable compensation for the year.

• Calculate and process any year-end bonus needed to satisfy reasonable comp.

• Confirm year-end payroll deposits and W-2 issuance schedules.

• For a more-than-2% S-corporation shareholder-employee, confirm that qualifying health-insurance premiums paid or reimbursed under a plan established by the S corporation are included in Form W-2 wages before year-end when required for the §162(l) self-employed health-insurance deduction. Shareholders at or below 2% generally follow employee fringe-benefit rules.

Pass-Through Entity Tax (PTET) Election

For S corporations and partnerships in PTET states, verify election eligibility, owner treatment, state deadlines, and entity-level estimated payments. Election and payment dates differ by state, and cash-basis federal deduction timing generally depends on when the entity pays the tax. Model the election against the owner's 2026 personal SALT limit and income phase-down.

December: Final Optimization

Charitable Giving

• Standard vs. itemized analysis: If close to the $32,200 standard deduction for joint filers, compare bunching with the new 2026 charitable-deduction rules, including the 0.5% AGI floor for itemizers and the limited cash deduction for non-itemizers.

• Donor-advised fund contributions for bunching with retained discretion over recipient timing.

• Long-term appreciated stock donations may avoid recognizing the embedded gain and support a fair-market-value deduction when the shares are qualifying capital-gain property, subject to AGI and substantiation limits.

• Qualified Charitable Distribution (QCD) for eligible IRA owners age 70½ or older — use a direct IRA-to-charity transfer and confirm the current annual indexed limit.

Tax-Loss Harvesting

For taxpayers with significant investment portfolios:

• Identify positions with unrealized losses that can offset realized gains.

• Wash sale rule awareness — a loss can be deferred when substantially identical stock or securities are acquired during the 30 days before or after the loss sale, creating a 61-day testing window; spouse and IRA acquisitions can also matter.

• Up to $3,000 of net capital losses can offset ordinary income annually; excess carries forward indefinitely.

Capital Gains Realization

For taxpayers in the 0% long-term capital gains bracket (taxable income up to $98,900 for joint filers in 2026), December can be an opportunity to harvest gains, but ordinary income, qualified dividends, and gains all affect how much of the 0% band remains.

Roth Conversions

Year-end Roth conversions allow filling lower brackets with conversion income. Critical: the conversion must be completed by December 31 — there is no grace period or extension. Coordinate with:

• Marginal rate analysis.

• IRMAA Medicare premium thresholds.

• NIIT MAGI thresholds.

• State tax exposure.

Quarterly Estimated Tax True-Up

The Q4 estimated tax payment is due January 15. Use the projected year-end income to true up estimated payments to satisfy safe harbor:

• 90% of current-year tax, OR

• 100% of prior-year tax, increased to 110% when prior-year AGI exceeded $150,000 ($75,000 if married filing separately).

Withholding (which is treated as paid evenly throughout the year) can sometimes cure underpayment penalties that an estimated payment cannot.

Required Minimum Distributions (RMDs)

Retirees age 73+ must take RMDs by December 31 (with one-time exception for age 73 first year — can defer to April 1 of following year). Missing the RMD triggers a 25% penalty (10% if corrected within 2 years under SECURE 2.0).

Annual Gift Exclusion Gifts

Each donor can give $19,000 in 2026 per recipient under the annual gift-tax exclusion without using lifetime exemption. For families using an annual-exclusion strategy, completed gifts must occur by December 31.

FSA Use-or-Lose Deadlines

For employees with health FSA balances, use remaining funds by December 31 or the plan's applicable grace-period date. A plan may permit a 2026 carryover up to $680; plan terms control.

Documentation and Recordkeeping

December is also the right time to:

• Reconcile bookkeeping through year-end.

• Ensure 1099-NEC and 1099-MISC information is collected for contractors.

• Verify W-9 forms are on file for all vendors requiring 1099s.

• Collect K-1s, 1099s, and other income documents as they arrive in January.

• Document mileage logs, accountable plan reimbursements, and home office calculations.

Multi-Year Strategy Considerations

Year-end planning should also consider multi-year positioning:

• Income smoothing across years to manage marginal rate exposure.

• Roth conversion ladder during lower-income years before required minimum distributions begin, with IRMAA and state-tax modeling.

• Cost segregation studies for newly acquired or owned commercial real estate.

• Estate planning gifts under the $15 million 2026 basic exclusion amount, coordinated with state estate-tax rules and non-tax objectives.

• QBI optimization under the permanent §199A rules, including 2026 thresholds, W-2 wages, UBIA, aggregation, and specified-service-business status.

Common Mistakes

• Waiting until December to begin planning (some strategies require Q3 execution).

• Missing the December 31 placed-in-service deadline for equipment.

• Failing to true up S-corp salary before year-end payroll closes.

• Roth conversions that push MAGI above IRMAA thresholds.

• Charitable giving without bunching analysis.

• Missing PTET election year-end payment requirements.

• Inadequate documentation of accountable plan reimbursements.

Bottom Line

Year-end tax planning is a practical time to coordinate projections, payroll, depreciation, retirement plans, PTET elections, estimated payments, and documentation before applicable deadlines pass. The value of any strategy depends on the taxpayer's facts, cash flow, state rules, and multi-year projections, so each move should be modeled before implementation.