December 31 is a hard deadline for employee salary deferrals into an existing 401(k), including a Solo 401(k) participant contribution. It is not the deadline for adopting most new plans or making employer contributions. The plan type and business structure decide which deadline applies. New plan adoption and employer contributions often can wait until the tax filing deadline, with or without extensions.
Table of Contents
- Which retirement plan deadlines actually fall on December 31?
- Can you set up a 401(k) after December 31?
- What is the Solo 401(k) contribution deadline for 2026?
- What is the SEP IRA and SIMPLE IRA deadline?
- Can you replace a SIMPLE IRA with a 401(k) before year end?
- How does Spencer Accounting Group handle retirement plan setup and catch-up filing?
- What should a business owner confirm before December 31, 2026?
- Key Takeaways
- References
Which retirement plan deadlines actually fall on December 31?
December 31 is the funding deadline for employee salary deferrals into an existing 401(k), including the Solo 401(k) participant contribution. The Solo 401(k) participant maximum is $24,500 for 2026 and $23,500 for 2025, according to Invesco. The catch-up contribution at age 50 is $8,000 for 2026 and $7,500 for 2025. The super catch-up for ages 60 through 63 is $11,250 for both 2026 and 2025.
Employer contributions and new plan adoption usually follow a different deadline. The two categories should not be conflated. A business owner who misses the December 31 employee deferral cutoff may still have time to adopt a plan or fund employer contributions later. Deadlines shift year to year and should be confirmed for the current tax year.
Can you set up a 401(k) after December 31?
Yes. SECURE 1.0 extended plan adoption from the last day of the tax year to the tax filing deadline plus extensions for 2020 and later tax years, according to Employee Fiduciary. For an S-Corporation or partnership, the traditional 401(k) adoption deadline is March 15, or September 15 with an extension. For a C-Corporation or sole proprietorship, the deadline is April 15, or October 15 with an extension.
A calendar-based safe harbor 401(k) is the exception. That plan must be adopted by October 1. The safe harbor rule exists because those plans carry mandatory employer contribution requirements that need a full plan year to operate correctly. Business owners considering a safe harbor design should treat October 1 as the operative date, not December 31.
What is the Solo 401(k) contribution deadline for 2026?
The participant deferral must be funded by December 31. The overall Solo 401(k) contribution limit is $72,000 for 2026 and $70,000 for 2025, funded by the tax filing deadline plus extension, according to Invesco. The new plan deadline is the tax filing deadline plus extension for incorporated businesses. For sole proprietors, the new plan deadline is the tax return deadline with no extensions.
SECURE 2.0 Section 317 changed one rule for sole proprietors and single-member LLCs. Those owners can make first-year employee contributions up to the tax return due date, determined without regard to extensions, effective for plan years beginning after December 29, 2022, according to Retirement Learning Center. That provision applies only to the first plan year and only to the employee deferral portion.
What is the SEP IRA and SIMPLE IRA deadline?
The SEP IRA employer contribution is $72,000 per participant for 2026 and $70,000 for 2025, funded by the tax filing deadline plus extension, according to Invesco. The SEP IRA new plan deadline is also the tax filing deadline plus extension. A SEP IRA can be adopted and funded after December 31 for the prior tax year.
SIMPLE IRA participant contributions must be funded within 7 business days of payroll. The SIMPLE IRA participant maximum is $18,100 for 2026 and $17,600 for 2025 for employers with 25 or fewer employees, and $17,000 for 2026 and $16,500 for 2025 for 26 or more employees. A SIMPLE IRA can be set up effective any date from January 1 through October 1, according to IRS Publication 560.
Can you replace a SIMPLE IRA with a 401(k) before year end?
Yes, in specific circumstances. SECURE 2.0 Section 332 allows a mid-year replacement of a SIMPLE IRA with a SIMPLE 401(k) or other 401(k) requiring mandatory employer contributions, effective for plan years beginning after December 31, 2023, according to Retirement Learning Center. For a December 31 SIMPLE IRA termination, participants must be notified at least 60 days in advance. For a mid-year termination, the notice window is at least 30 days.
The prior rule required notifying participants by November 2, 2024 to terminate on December 31 and start a 401(k) on January 1, 2025, according to Employee Fiduciary. That notice deadline was specific to the 2024 to 2025 transition. Business owners planning a future replacement should calculate the notice window from their intended termination date.
How does Spencer Accounting Group handle retirement plan setup and catch-up filing?
Spencer Accounting Group is a 100% virtual accounting firm founded in 2013 serving business owners in any state or country, with no office visit required. The firm's Strategic Tax Planning service designs the tax position around where the business is heading rather than discovering it in April. Retirement plan selection fits into that forward-looking work because the plan type changes which deadlines and contribution limits apply.
Bookkeeping keeps clean monthly books year round so tax time holds no surprises and the numbers can actually be used to make decisions. Tax Filing prepares and files business and individual returns through a secure portal. For owners who are behind, Back Tax Return Filing handles catch-up filing without judgment and sequences the work to reach compliance with the least disruption.
What should a business owner confirm before December 31, 2026?
Confirm which deadline applies to the plan type and entity structure. A Solo 401(k) participant deferral follows the December 31 funding rule, while a SEP IRA employer contribution follows the tax filing deadline plus extension. Confirm whether the plan is new or existing, since adoption and funding deadlines differ. Confirm the current year's contribution limits, since they shift year to year.
Confirm whether an extension is available and whether it has been filed. The extension changes the adoption and employer contribution deadline for many plan types. This article is general information, not tax advice for a specific situation. A business owner with a multi-state operation or a history of unfiled returns should have the plan decision reviewed in the context of the full tax position.
Key Takeaways
- Employee deferrals into an existing 401(k), including a Solo 401(k), must be funded by December 31.
- The Solo 401(k) participant maximum is $24,500 for 2026 and $23,500 for 2025.
- SECURE 1.0 moved new plan adoption to the tax filing deadline plus extensions for 2020 and later tax years.
- A safe harbor 401(k) must be adopted by October 1.
- A SIMPLE IRA can be set up effective any date from January 1 through October 1.
- SEP IRA employer contributions are funded by the tax filing deadline plus extension.
- Deadlines shift year to year and should be confirmed for the current tax year.
References
- Retirement contribution limits and deadlines — Invesco
- Key 2025 Retirement Deadlines You Need to Know — TrustETC
- Deadlines for 401(k) Adoption – Including SECURE 2.0 Changes — Employee Fiduciary, January 31, 2024
- How SECURE 2.0 Effects Plan Establishment Deadlines — Retirement Learning Center, February 28, 2023
- Publication 560 (2025), Retirement Plans for Small Business — IRS