Solo 401(k) contribution deadlines for 2026: employee and employer

A solo 401(k) has two deadlines, not one, and they fall on different dates for different reasons. The employee half is governed by when you decide to contribute; the employer half by when you pay. Miss the first and no amount of money in April fixes it. This current guide (the address dates from an earlier edition) covers who qualifies, the 2026 limits, each deadline by entity type, the SECURE 2.0 first-year exception, Roth options, and a worked example for a contractor netting $180,000.

Who can use a solo 401(k)

The IRS calls it a one-participant 401(k): an ordinary 401(k) covering a business owner with no employees, or the owner and a spouse. That covers contractors with a single-member LLC, consultants with an S corporation, and engineers with a W-2 job and a paid side project. Hire a non-spouse employee who meets the plan's eligibility rules and the plan must cover them.

One limit follows the person, not the plan: the elective deferral cap applies across every 401(k) you participate in during the year. If you defer $20,000 into your employer's plan (see our Amazon 401(k) guide for how a large plan works), only $4,500 of deferral room is left for the solo plan in 2026. The employer half is separate for each unrelated business, so the solo plan stays worthwhile for side income.

The two halves of a contribution

• Employee elective deferral. You, as employee, choose to put part of your pay into the plan, pre-tax or Roth. Up to 100% of compensation, capped by the annual deferral limit.
• Employer contribution. The business contributes on your behalf. For an owner paid on a W-2 (S or C corporation) the cap is 25% of W-2 compensation. For a sole proprietor or partner the same 25% rule is applied to net earnings after the contribution itself and after half of self-employment tax, which the Publication 560 rate table reduces to an effective 20% of net earnings.

The two halves together cannot exceed the overall annual additions limit, which, per the IRS, does not count catch-up contributions.

2026 limits

From IRS Notice 2025-67, with the Social Security wage base from Publication 15:

Item	2026
Employee elective deferral, section 402(g)	$24,500
Catch-up, age 50 and over	$8,000
Higher catch-up, ages 60 to 63	$11,250 (replaces the $8,000, not in addition to it)
Total annual additions, section 415(c), excluding catch-up	$72,000
Compensation that can be counted, section 401(a)(17)	$360,000
Roth catch-up wage threshold (prior-year FICA wages)	$150,000
Social Security wage base (for the self-employment tax step)	$184,500

So a contractor under 50 can put in up to $72,000 in 2026; at 50 or older, $80,000; at 60 to 63, $83,250. SECURE 2.0 requires catch-ups to be Roth when prior-year FICA wages exceeded the threshold; Treasury's final regulations (T.D. 10033) confirm self-employment income is not FICA wages, so this binds S corporation owners with large W-2 wages, not sole proprietors or partners.

Deadlines for the employee half

The December 31 election rule

Under Treasury Regulation 1.401(k)-1(a)(6)(iii), a sole proprietor's or partner's compensation is deemed available on the last day of the taxable year, and "a self-employed individual may not make a cash or deferred election with respect to compensation for a partnership or sole proprietorship taxable year after the last day of that year." In plain terms: by December 31, 2026 you must have decided, in writing, how much of your 2026 earnings you are deferring. The money itself can follow later; under Publication 560, contributions count for a year if made by the return due date including extensions, provided the plan existed by year-end.

If you pay yourself a W-2 (S or C corporation), the deferral must come out of actual payroll during the year. Once the last 2026 payroll runs, the window closes. Department of Labor rules then require the withheld amounts to reach the plan as soon as they can reasonably be segregated; for plans with fewer than 100 participants, a deposit within seven business days is deemed timely under 29 CFR 2510.3-102.

The SECURE 2.0 first-year exception

Section 317 of SECURE 2.0 created a narrow exception. Publication 560 states it this way: for 2023 and later, "a sole proprietor with no employees can adopt a section 401(k) plan after the end of the tax year, provided the plan is adopted by the tax filing deadline (without regard to extensions)," and the statute treats a deferral election made by that date as made by the prior December 31 for the plan's first year. A contractor who never opened a plan in 2026 can therefore open one by April 15, 2027, elect a 2026 deferral, and fund it. It covers sole proprietors (including single-member LLCs), only the first plan year, and only to the unextended due date; partnerships and corporations do not get it.

Deadlines for the employer half, and for opening the plan

The employer contribution is a deduction question, and Publication 560 is direct: "You can make deductible contributions for a tax year up to the due date of your return (plus extensions) for that year." Since the original SECURE Act, an employer may adopt a plan after year-end and treat it as effective for the prior year, so long as the plan is adopted by the return due date including extensions; the IRS explains the rule in an issue snapshot. That retroactive adoption works for any entity type, but it only supports the employer half; the employee half still needs the December 31 election unless the first-year exception applies.

Deadlines by entity type

For the 2026 tax year and calendar-year businesses, using the due dates from IRS Publication 509:

Entity	Return and due date	Employee deferral election	Employee deferral deposit	Employer contribution and retroactive plan adoption
Sole proprietor or single-member LLC (Schedule C)	Form 1040, April 15, 2027; extended to October 15, 2027	December 31, 2026, or April 15, 2027 for a plan's first year under SECURE 2.0 section 317	By the return due date including extensions	April 15, 2027, or October 15, 2027 on extension
Partnership or multi-member LLC	Form 1065, March 15, 2027; extended to September 15, 2027	December 31, 2026	By the partnership return due date including extensions	March 15, 2027, or September 15, 2027 on extension
S corporation	Form 1120-S, March 15, 2027; extended to September 15, 2027	Before each payroll; last chance is the final 2026 payroll	Within seven business days of withholding (small-plan safe harbor)	March 15, 2027, or September 15, 2027 on extension
C corporation	Form 1120, April 15, 2027; extended to October 15, 2027	Before each payroll; last chance is the final 2026 payroll	Within seven business days of withholding	April 15, 2027, or October 15, 2027 on extension

What an extension does and does not extend

Filing Form 4868 (individuals) or Form 7004 (partnerships and corporations) gives six more months to file, and because the employer contribution deadline is "the due date including extensions," it moves that deadline too. An extension does not extend the December 31 election rule, and does not extend the first-year exception, which Publication 560 ties to the due date "without regard to extensions." If you are relying on section 317, April 15, 2027 is a hard stop even if you file in October.

Roth solo 401(k) and Roth employer contributions

Most solo plan documents allow the employee deferral to be designated Roth, taxed now and tax-free later, with no income limit. SECURE 2.0 section 604 added the option to designate the employer contribution as Roth as well. Under IRS Notice 2024-2, a Roth employer contribution is included in your income in the year it is allocated to your account, even if it is deposited in 2027 and deducted for 2026; it is reported on Form 1099-R rather than a W-2, is not subject to withholding, and must be fully vested, which an owner's contribution always is. Roth 401(k) balances also have no lifetime required minimum distributions. If your plan document permits after-tax (non-Roth) contributions and in-plan conversions, the gap between your deferral and the $72,000 ceiling can be filled that way; our mega backdoor Roth calculator shows the math, and the high-earner guide puts it in order with the other moves.

Form 5500-EZ

A one-participant plan files no annual return until its assets exceed $250,000 at the end of a plan year (Publication 560; IRS one-participant page). Once they do, Form 5500-EZ is due by the last day of the seventh month after the plan year ends, July 31 for a calendar-year plan, with an extension available on Form 5558 (IRS Form 5500 Corner). A final return is also required in the year the plan is terminated regardless of size.

Worked example: $180,000 of net self-employment income

A 45-year-old contractor in Denver files Schedule C with $180,000 of net profit for 2026 and opened her solo 401(k) in 2025. No other 401(k).

Step	Amount
Net profit from Schedule C	$180,000.00
Net earnings subject to self-employment tax (92.35%)	$166,230.00
Social Security tax, 12.4% (below the $184,500 base)	$20,612.52
Medicare tax, 2.9%	$4,820.67
Total self-employment tax	$25,433.19
Deductible half of self-employment tax	$12,716.60
Net earnings for the plan ($180,000 less $12,716.60)	$167,283.40
Maximum employer contribution, 20%	$33,456.68
Employee deferral (elected by December 31, 2026)	$24,500.00
Total contribution	$57,956.68
Room left under the $72,000 ceiling	$14,043.32

If she were 50 she could add an $8,000 catch-up for $65,957; at 60 to 63, $11,250 for $69,207. Her deadlines: sign the deferral election by December 31, 2026; deposit both halves by April 15, 2027, or by October 15, 2027 if she extends her return. Had she opened the plan in February 2027 instead, section 317 would still let her elect and fund the 2026 deferral, but only until April 15, 2027. The $57,957 reduces 2026 taxable income out of the 24% federal bracket, saving roughly $13,700 federally plus about $2,550 of Colorado tax at the flat 4.40% described in our Colorado income tax guide.

Common mistakes

1. Deciding the deferral amount in April. For an existing plan, the election had to be made by December 31. The first-year exception is the only exception.
2. Counting the day-job deferral twice. The $24,500 is per person across all plans; only the employer half is separate.
3. Using 25% instead of 20% on Schedule C income. The 25% applies to W-2 wages; for self-employment earnings the Publication 560 worksheet reduces it to 20% of net earnings after half of self-employment tax.
4. Treating catch-ups as part of the $72,000. They sit on top of it.
5. Assuming an extension extends everything. It extends the employer contribution, not the election or the first-year exception.
6. S corporation owners paying themselves a token salary. Both halves are capped by W-2 wages, so a $40,000 salary caps the employer half at $10,000.
7. Missing Form 5500-EZ at $250,000. Set the reminder.

Limits are indexed annually and SECURE 2.0 guidance keeps arriving; confirm the current figures in the IRS limits notice and Publication 560 before timing a contribution.

Related guides

• The comprehensive guide to your Amazon 401(k)
• How often do 401(k)s compound, and how to think about investing within your 401(k)
• Intel SERPLUS: a complete guide
• Retired tech professionals: planning the years between the last paycheck and Social Security