Can I Still Open a Solo 401(k) for Last Year?

Published: September 7, 2026 · Reading time: 9 min

TL;DR: Yes, and the answer most guidance gives — "you had to open it by December 31" — has been wrong since 2019. IRC §401(b)(2) now holds two different deadlines. Adopt the plan and make employer contributions any time up to your filing deadline including extensions (October 15 if you extend). Make elective deferrals, for a first plan year only, up to your filing deadline determined without regard to extensions (April 15) — that second sentence was added by SECURE 2.0 §317 and it is the piece nearly everyone misses. On $110,000 of net profit, the deferral is worth $24,500 of extra deduction and $4,312.00 of federal tax at a 22% margin — not the $5,390 the sticker arithmetic suggests, because a retirement deduction also shrinks your QBI deduction. Extending your return does not extend the deferral window, and none of it repeats in year two.

It's March. You had a good year, you did not open a retirement plan, and everything you read says you needed to do that by December 31.

That advice describes the law as it stood before 2019.

What §401(b)(2) Actually Says

The statute is short and it contains two distinct rules. Here is the first sentence:

If an employer adopts a stock bonus, pension, profit-sharing, or annuity plan after the close of a taxable year but before the time prescribed by law for filing the return of the employer for the taxable year (including extensions thereof), the employer may elect to treat the plan as having been adopted as of the last day of the taxable year.

That is the SECURE Act of 2019 change. It is why a December 31 adoption deadline is no longer the rule for anyone.

And here is the second sentence, added by SECURE 2.0 §317:

In the case of an individual who owns the entire interest in an unincorporated trade or business, and who is the only employee of such trade or business, any elective deferrals (as defined in section 402(g)(3)) under a qualified cash or deferred arrangement to which the preceding sentence applies, which are made by such individual before the time for filing the return of such individual for the taxable year (determined without regard to any extensions) ending after or with the end of the plan's first plan year, shall be treated as having been made before the end of such first plan year.

Read those two sentences side by side and three things fall out:

Extending your return extends one column and not the other. That is not an oversight in how the article is written; it is what the statute says, and it is the single most expensive misreading in this area.

Where the December 31 advice comes from — including on this site

To be straight about it: several older posts here state a flat December 31 deadline for solo 401(k) deferrals, and one of them goes further and says SECURE 2.0 allows post-year-end opening for "employer-only" contributions. Two things are wrong in that framing. The retroactive-adoption rule for employer contributions came from SECURE 1.0 §201 in 2019, not from SECURE 2.0. And what SECURE 2.0 §317 actually did was create the very deferral window that sentence denies.

If you are reading an article on this site or elsewhere that gives you December 31 without qualification, read it as describing a plan's second and later years, where it is correct.

Why It Is First-Year Only

The scope limit is not arbitrary. An elective deferral is, by definition, a deferral of compensation you have not yet received — you cannot elect to defer money that is already in your hands.

For a self-employed person, earned income is treated as received on the last day of the taxable year. So in the ordinary case the deferral election has to exist by December 31, because on January 1 the compensation has been constructively received and there is nothing left to defer.

§317 carves out a narrow exception for someone who did not have a plan at all during the first year, because there was no arrangement in which to make an election. Once the plan exists, the ordinary logic resumes.

Practical consequence: the year you set up the plan you get a grace period. Every year after that, December 31 is real for the deferral and October 15 is real for the employer contribution. Put both in the calendar.

It Does Not Work for an S Corporation

§317 is written for "an individual who owns the entire interest in an unincorporated trade or business, and who is the only employee of such trade or business."

An S corporation is not an unincorporated trade or business, and its owner is a W-2 employee. Employee deferrals have to be withheld from wages actually paid during the plan year, so an S corp owner who did not run deferrals through December payroll cannot manufacture them in April.

The employer side still works: the corporation can adopt a plan and make employer contributions up to the extended filing deadline under the first sentence. This is one of the less-discussed frictions in the S corp election decision — the payroll discipline an S corp requires is real, and it extends to retirement deferrals.

Worked Example: $110,000 of Net Profit, Discovered in March

Marcus is a sole proprietor with $110,000 of Schedule C net profit for 2026. He is 41, has no employees, has never had a retirement plan, and reads this in March 2027 — before his unextended filing deadline. He files jointly; his spouse's W-2 wages of $105,000 keep the household inside the 22% bracket for 2026 (taxable income over $100,800 and not over $211,400) both before and after every contribution below — so 22% is genuinely his marginal rate all the way down the column, rather than a rate that quietly breaks partway.

Step 1 — self-employment tax. $110,000 × 92.35% = $101,585.00, × 15.3% = $15,542.51. Half of that, $7,771.26, is his above-the-line deduction. (He is comfortably under the 2026 Social Security wage base of $184,500, so the full 15.3% applies.)

Step 2 — the employer contribution base. $110,000.00 − $7,771.26 = $102,228.74.

Step 3 — the employer contribution. The 25%-of-compensation limit becomes 20% of that figure for a sole proprietor, because plan compensation is itself net of the contribution: $102,228.74 × 20% = $20,445.75.

Step 4 — the elective deferral. The 2026 §402(g) limit is $24,500. Marcus is under 50, so no catch-up.

Step 5 — check the ceilings. His plan compensation, or earned income, is $102,228.74 − $20,445.75 = $81,782.99, and the deferral is well under 100% of that. Total annual additions are $20,445.75 + $24,500.00 = $44,945.75, comfortably inside the 2026 §415(c) cap of $72,000.

Step 6 — the part almost every calculator skips. A retirement contribution does not reduce your taxable income dollar for dollar, because the deduction for self-employed retirement contributions itself reduces qualified business income. The IRS lists it alongside the deductible half of SE tax and self-employed health insurance among the items that reduce QBI. So every dollar contributed also shaves 20 cents off the §199A deduction, and only 80 cents of it actually leaves taxable income.

The §317 deferral window is the difference between the last two rows: $24,500.00 of extra deduction, worth $4,312.00.

Three honest caveats on that number, because it is smaller than the one most articles quote.

1. It is not $5,390. Multiplying $24,500 by 22% is the arithmetic everyone does, and it overstates by $1,078 for exactly the reason in Step 6 — the QBI deduction falls by $4,900 at the same time.
2. Retirement contributions do not reduce self-employment tax. The $15,542.51 is identical in every row. Only the income tax moves.
3. It is a deferral, not forgiveness. The value is the rate arbitrage between contributing now and withdrawing later, plus decades of untaxed compounding — not the $4,312.00 by itself.

A note on the 22%. Marcus's marginal rate is a fact about his household, not about his $110,000 of profit. A single filer with that same Schedule C and no other income lands in the 12% bracket for the last slice of the deduction, where the deferral is worth materially less. Any article that quotes you a single dollar figure without telling you whose bracket it assumed — including the older ones on this site — is quoting you a number that belongs to someone else.

If Marcus reads this in June instead, the deferral is gone. He extends, adopts the plan by October 15, contributes the $20,445.75 employer piece for 2026 — and the practical question becomes whether a solo 401(k) or a SEP-IRA is the better vehicle, since for employer-only money they reach the same number and the SEP is simpler.

What "Retroactive" Does and Does Not Get You

The word does a lot of work in marketing copy, so here is the boundary.

It does get you:

• A plan treated as adopted on the last day of the prior taxable year
• Employer nonelective contributions deducted on the prior year's return
• For a first plan year, elective deferrals treated as made before that year ended

It does not get you:

• Deferrals in the second or any later plan year
• Anything at all for an S corporation owner's employee deferrals
• A retroactive catch-up where you were not the qualifying age in the prior year
• Relief from having actually adopted a plan document — a contribution to a plan that does not exist is not a contribution, it is a taxable investment in a brokerage account
• Relief from the ordinary limits: $24,500 deferral, §415(c) annual additions of $72,000, and 100% of earned income are all unchanged

On Roth: the statute refers to "elective deferrals (as defined in section 402(g)(3))," which is not limited to pre-tax deferrals, and the practitioner reading is that a retroactive first-year deferral can be designated Roth. We could not confirm that from a primary IRS source, and not every solo 401(k) plan document supports designated Roth deferrals in the first place — so treat it as a question for your provider rather than a settled answer. If it matters to you, ask before you sign the adoption agreement, not after. Our Roth vs. traditional solo 401(k) guide covers the underlying choice.

The Paperwork a One-Person Plan Still Needs

Nobody else is generating this for you, which is exactly why it goes missing.

• The plan document and adoption agreement, signed and dated on or before the deadline you are relying on. Keep the signed copy, not just the provider's portal confirmation.
• A written deferral election with a date, the amount or percentage, whether it is pre-tax or Roth, and the plan year it applies to. For a first-year retroactive deferral, dated by the unextended deadline.
• An EIN for the plan sponsor. Our EIN vs. SSN guide covers when a sole proprietor needs one.
• Your net profit and SE tax computation, since plan compensation is derived from them. The IRS defines a self-employed person's compensation as "earned income," which is "net earnings from self-employment after deducting both: one-half of your self-employment tax, and contributions for yourself."
• Form 5500-EZ once the plan has $250,000 or more in assets at the end of the year. Below that there is generally no annual return, which is one of the quiet advantages of a one-participant plan — and one of the easiest obligations to trip over the year you cross it.

Where This Sits in the Calendar

Filing an extension is nearly free and it preserves the October column — see our Form 4868 guide. Just do not let it create the impression that everything moved. The April column does not move.

Frequently Asked Questions

Can I open a solo 401(k) after December 31 and contribute for last year?

Yes. §401(b)(2)'s first sentence lets an employer adopt a plan up to the return filing deadline "including extensions thereof" and elect to treat it as adopted on the last day of the prior taxable year — October 15 for a calendar-year filer who extends. The December 31 rule was pre-2019 law.

What is the deadline for the employee deferral portion?

For a first plan year and a sole proprietor who is the only employee: the filing deadline without regard to extensions, so April 15. That is SECURE 2.0 §317, the second sentence of §401(b)(2). Extending your return does not extend it.

Does the retroactive deferral rule apply in the plan's second year?

No — §317 is scoped to "the plan's first plan year." From year two the ordinary rule returns: a self-employed person's earned income is treated as received on the last day of the year, so the deferral election must be in place by December 31. Employer contributions keep the extended deadline every year.

Does this work for an S corporation owner too?

No. §317 covers an individual owning the entire interest in an unincorporated trade or business who is its only employee. An S corp owner is a W-2 employee whose deferrals must come out of wages paid during the year. The employer contribution side still runs to the extended deadline.

How much can I actually contribute retroactively?

For 2026: deferrals up to $24,500 ($8,000 age-50 catch-up, $11,250 at 60–63), plus an employer contribution of 20% of net profit minus half of SE tax, with total annual additions capped at $72,000 under §415(c). On $110,000 of net profit: $20,445.75 employer plus $24,500 deferral = $44,945.75. At a 22% margin the deferral half is worth $4,312.00 — not $5,390, because the retirement deduction also cuts your QBI deduction by $4,900.

Should I just do a SEP-IRA instead?

It is the right fallback once the deferral window has closed — same extended deadline, no plan document, no Form 5500-EZ — but it has no deferral component, so it stops at the employer-side figure. On $110,000 of net profit that is $20,445.75 against the solo 401(k)'s $44,945.75.

Authoritative References

• Cornell LII / U.S. Code — 26 U.S.C. §401, subsection (b)(2), both sentences
• SECURE 2.0 Act of 2022, §317 — section-by-section summary
• IRS — One-participant 401(k) plans: the earned-income definition and the $250,000 Form 5500-EZ threshold
• IRS — 401(k) and profit-sharing plan contribution limits: 2026 §402(g) and §415(c) figures
• IRS — Publication 560: Retirement Plans for Small Business, Chapter 5, for the self-employed contribution computation
• IRS — Notice 2024-2: miscellaneous SECURE 2.0 guidance

Related reading: 2026 Schedule C numbers · SEP-IRA vs. solo 401(k) · Solo 401(k) contribution limits · Roth vs. traditional solo 401(k) · SIMPLE IRA for freelancers · Catch-up contributions over 50 · Tax extension Form 4868 · Freelancer tax deadlines calendar 2026 · S corp election · Schedule C EIN vs. SSN

The Deadline You Can Only Use If You Know the Number

Every figure above starts from one place: your net profit. A March scramble to reconstruct a year of expenses is how a $4,312 deferral window gets missed — not because the rule was unknown, but because the number wasn't ready in time to act on it. CentSense keeps net profit current as the year runs, so in January you already know what you can contribute instead of finding out in October. Free tier includes 10 AI scans per month; Solo is $5/month for unlimited scanning and automatic mileage logging.

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This guide is general education for U.S. freelancers and independent contractors filing for the 2026 tax year. It is not personalized tax or investment advice. The designated-Roth treatment of a retroactive first-year deferral is flagged above as unconfirmed against primary IRS guidance; confirm it with your plan provider and a CPA or EA before relying on it.