How late can you still open a retirement account for last year?

One of the few genuinely useful things about self-employed retirement plans is that some of them let you make a decision about a year that has already ended. Not all of them, and the differences are not intuitive.

The short answer: a SEP-IRA is the one you can still open after the fact. Everything else has a catch.

The four cut-offs

Plan	Open by	Fund by	Does an extension help?
SEP-IRA	Tax filing deadline incl. extensions	Same	Yes
Traditional / Roth IRA	15 April (unextended)	15 April (unextended)	No
Solo 401(k) — employer part	Tax filing deadline incl. extensions	Same	Yes
Solo 401(k) — employee deferral	Generally 31 December	Filing deadline	Usually not

The SEP-IRA: genuinely retroactive

You can establish a SEP-IRA and fund it right up to the due date of the return, including extensions, and have it count for the prior year.

For tax year 2025 that means 15 October 2026 if you filed an extension. In August or September of 2026 you can still open an account that did not exist during 2025 and reduce your 2025 tax bill with it.

This is unusual and worth knowing, because it is the only retirement decision on this list that survives contact with hindsight. You already know your actual profit, so you can size the contribution against a real number rather than a forecast.

The IRA: the deadline that does not move

A traditional or Roth IRA contribution for a tax year must be made by the unextended filing deadline — 15 April — even if you extended your return to October.

This trips up people who assume every deadline moves together. It does not. Filing an extension buys you six months for the return and zero extra days for the IRA.

The solo 401(k): two deadlines, not one

A solo 401(k) has two kinds of money going in, and they are treated differently.

The employer contribution — the same roughly-20% employer percentage a SEP uses — follows the SEP rule. The SECURE Act allows a plan to be adopted as late as the filing deadline including extensions and still receive an employer contribution for that year.

The employee deferral is the awkward one. It is a deferral of compensation, which normally has to be elected before the compensation is earned — so for an ongoing plan, the election is due by 31 December, with the money itself payable by the filing deadline.

There is one narrow escape hatch. SECURE 2.0 lets a sole proprietor with no employees who opens a solo 401(k) for the first plan year make employee deferrals up to the unextended filing deadline. It applies to the first year only, and it is easy to misread as a general rule. It is not.

The practical read: if you want the deferral — and the deferral is the entire reason a solo 401(k) beats a SEP — open the plan before 31 December. Opening it costs nothing and funding decisions can come later.

Which one to use when you have run out of year

If it is already the following spring and you want to shelter income from the closed year:

• You extended and it is before mid-October: a SEP-IRA works, from a standing start.
• It is before 15 April: a SEP works, and an IRA is also still open to you. They are not mutually exclusive — an IRA contribution is separate from the SEP limit, subject to its own rules on deductibility if you are covered by a workplace plan.
• You want a solo 401(k)‘s bigger number: for the closed year you can generally only get the employer slice, unless this is the first plan year and you are a sole proprietor. Open it now for the current year regardless.

Do not forget which year you are funding

Custodians make this mistake easy: a contribution made in March defaults to the current year unless you tell them otherwise. Designate the year explicitly when you send the money, and check the confirmation.

Getting it wrong is recoverable but tedious — it usually means a corrected form from the custodian, and if you have already filed, an amended return.

If the year is already closed and you have filed

You can still amend. Contributing to a SEP after filing but before the extended deadline is legitimate; you file a 1040-X to claim the deduction. Whether it is worth the effort depends on the size of the contribution and your marginal rate — the calculator below gives you the deduction, and your marginal rate tells you what that is actually worth.

The site’s prior-year calculators run the closed year’s rules, which is the right basis for that decision.