Year-End 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 2025 (joint filers), the bracket boundaries:

• 22% bracket up to $206,700.

• 24% bracket up to $394,600.

• 32% bracket up to $501,050.

• 35% bracket up to $751,600.

• 37% above.

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 §199A threshold ($241,950 single / $483,900 joint for 2025). Income near or above the threshold creates planning opportunities to reduce income through retirement contributions, charitable giving, or other deductions to preserve the QBI deduction.

November: Action and Execution

Retirement Plan Optimization

• Solo 401(k) employee deferral: Maximize the $23,500 limit by year-end (or $31,000 with age 50+ catch-up; $34,750 with SECURE 2.0 super-catch-up at 60-63).

• SEP-IRA / Solo 401(k) employer contribution: Calculate maximum 25% of compensation contribution; payment can be made by tax return due date (with extension).

• Defined Benefit Plan: For high-income solo professionals or small businesses, a DB plan can absorb $200,000+ in contributions. Plan must be ADOPTED by year-end (cash-basis taxpayers) though contributions can be made by tax return due date.

• HSA contributions: Maximize the $4,300 self / $8,550 family limit (plus $1,000 catch-up at 55+).

Equipment and Capital Expenditures

Decide on year-end capital purchases:

• Section 179 elective expensing up to $1,250,000 (limited by taxable income).

• Bonus depreciation at 40% for 2025.

• Heavy SUV §179 sub-limit: $31,300 for 2025.

• 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.

• Add health insurance to W-2 wages for shareholders before year-end (required for §162(l) self-employed health insurance deduction).

Pass-Through Entity Tax (PTET) Election

For S-corps and partnerships in PTET states, verify the election is in place and entity-level estimated tax payments are sufficient. Late November is the critical window to make the final Q4 estimated PTET payment to ensure full federal deductibility.

December: Final Optimization

Charitable Giving

• Standard vs itemized analysis: If close to the $30,000 standard deduction (joint), consider bunching charitable giving into a single tax year.

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

• Appreciated stock donations bypass capital gains tax while providing FMV deduction.

• Qualified Charitable Distribution (QCD) for IRA owners 70½ or older — direct IRA-to-charity transfers up to $108,000 (2025).

Tax-Loss Harvesting

For taxpayers with significant investment portfolios:

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

• Wash sale rule awareness — cannot rebuy substantially identical security within 30 days.

• 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 under $96,700 joint for 2025) — typically retirees and early retirees — December is the time to harvest gains at no federal tax cost, resetting basis at higher levels.

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 (110% if prior AGI exceeded $150K).

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 (2025) per recipient per year tax-free without consuming lifetime exemption. For families using annual exclusion strategy, gifts must be made by December 31.

FSA Use-or-Lose Deadlines

For employees with health FSA balances, use remaining funds by December 31 (or by the plan's grace period or carry-over date if applicable). Limited carry-over of $660 (2025) available if the plan permits.

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 for retirees in the 60-72 window.

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

• Estate planning gifts using the higher exemption before potential 2026 sunset.

• QBI optimization before potential §199A sunset after 2025.

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 the most concentrated period of opportunity in the annual tax cycle. The 90-day window from October through December produces more tax savings per hour invested than any other period of the year. Working with a CPA on a structured year-end review — typically October for projection, November for execution, and December for optimization — typically produces tax savings many times the cost of the engagement. For small business owners and high-earning individuals, year-end planning should be a non-negotiable annual activity.