Year-End Tax Planning for S-Corps and Partnerships: Deadlines, Extensions, and Retirement Contributions

April 15th is behind us, tax season is over and I’m already hearing about the same issues: 

missed retirement deadlines, forgotten Section 179 purchases, and confusion over which deadlines are flexible and which aren’t. So let’s get clear on what has to happen before December 31st.

Quick Answer: S-corp and partnership owners face three immovable year-end deadlines — employee 401(k) deferrals (December 31), Q4 estimated taxes (January 15), and Section 179 equipment in service (December 31). Employer profit-sharing contributions and tax filings can be extended, but employee deferrals cannot

What Year-End Tax Deadlines Can S-Corps and Partnerships Not Move?

There are several year-end deadlines you can’t move. I’m focusing on the three that hit S-corp and partnership owners the hardest.

First: Employee retirement deferrals.

If you pay yourself W-2 wages, your 401(k) contributions must come out of paychecks processed by December 31st. For 2026, that’s up to $24,500, plus catch-up if you’re over 50. You can’t retroactively defer in January. If you want to max out, you need to adjust your final payroll runs before year-end.

Second: Fourth-quarter estimated taxes.

That payment is due January 15th. Even with an extension, it’s still required to avoid penalties.

Third: Equipment purchases for Section 179.

To deduct it this year, equipment must be purchased, delivered, and in service by December 31st. Ordering isn’t enough—it must be operational.

What Year-End Tax Deadlines Can S-Corps and Partnerships Extend?

Now here’s what’s flexible. 

Employer retirement contributions.
Profit-sharing contributions can be made up until your tax filing deadline, including extensions. If you extend to October 15th, you have until then to fund and deduct it. This is completely different from employee deferrals, which lock on December 31st.

Tax filing extensions.
Partnerships and S-corps file by March 16 and can extend to September 15. Just remember—extensions give you more time to file, not more time to pay. 

Year-End Tax Planning Checklist for S-Corps and Partnerships

• Make sure you’ve paid in close to what you’ll owe in estimated taxes
• Adjust final payroll to max out 401(k) deferrals
• Get equipment purchased and in service before December 31st
  

Year-end tax planning is easier when payroll, retirement, and tax strategy are aligned. At INFINITI HR, we work directly with your CPA to make sure nothing gets missed, plan now, and December becomes a strategy session — not a fire drill. For more on current employment trends, check out our blog at infinitihr.com.

Want more on current employment trends?

Check out the recent blog, How to Align HR, Finance, and Operations Around Shared Business Goals or come back for additional pieces on human resources, payroll, insurance, and benefits.

 

*This article is for informational purposes only and does not constitute tax or legal advice. Consult your CPA or tax advisor for guidance specific to your situation