What You Can Still Fund Before October 15 (and What Closed in April)

If you filed an extension, you have until October 15 to file your 2025 return. A lot of people assume that means they also have until October 15 to make every tax-saving move they skipped in the spring. They don't. Some doors are still open. Most closed on April 15.

This post sorts the accounts into two piles, explains why the deadlines differ, and runs the numbers on the one move that still matters for most people reading this.

Why two different deadlines exist

The tax code has two separate timing rules for retirement money, and they don't match.

Personal accounts (Traditional IRA, Roth IRA, HSA) follow the rule in IRC §219(f)(3). A contribution counts for the prior year only if it's made by the due date of your return, not including extensions. For 2025 that was April 15, 2026. Pub 590-A says it plainly: "For most people, this means that contributions for 2025 must be made by April 15, 2026." Pub 969 says the same for HSAs. An extension does nothing for these.

Employer plans (SEP, 401(k) employer contributions, SIMPLE employer contributions) follow IRC §404(a)(6). A contribution made by the due date of the business return, including extensions, is treated as if it was made on the last day of the year. Pub 560 states the SEP deadline as the "due date of employer's return (including extensions)."

So the question is never "am I on extension?" The question is "which return is the deduction going on, and when is that return due?"

Which return, and when

This is where most of the confusion lives.

Schedule C, single-member LLC, sole proprietor. Your business return is your Form 1040. On extension, that's due October 15, 2026. The employer side is still open.

C corporation. Form 1120. A calendar-year C corp on extension is also due October 15. Still open.

S corporation or partnership. Form 1120-S or Form 1065. Extended returns were due September 15, 2026. That deadline passed two weeks ago, and the employer contribution deadline passed with it. If your S corp did not fund its SEP or profit-sharing by September 15, the 2025 deduction is gone. A personal 1040 extension to October 15 does not reopen it, because the deduction was never on your 1040. It belonged on the 1120-S.

If you're an S corp owner reading this, the useful takeaway is for 2026. Put the employer contribution on the calendar for early September next year, or better, fund it before you file in March.

Pile one: still open through October 15

These are employer contributions. They deduct on the business return, not as a personal line item. Figures are the 2025 limits from IRS Notice 2024-80.

SEP IRA. The lesser of 25% of compensation or $70,000. A SEP can even be set up after year end, as long as it's set up and funded by the extended due date of the return. That makes it the simplest move on this list.

401(k) employer profit-sharing. The plan's total annual additions (your deferrals, employer match, employer nonelective, and forfeitures combined) are capped at the lesser of $70,000 or 100% of compensation. So the room left for a profit-sharing contribution is $70,000 minus whatever already went in during 2025.

SIMPLE IRA employer side. Either a dollar-for-dollar match up to 3% of compensation, or a flat 2% nonelective contribution for every eligible employee. The 2% version is computed on compensation up to $350,000 for 2025, so it tops out at $7,000 per person.

Pile two: closed since April 15

Traditional IRA. $7,000, or $8,000 if you were 50 or older by the end of 2025. Deadline was April 15. Even when it's timely, the deduction phases out if you're covered by a workplace plan. For 2025 that's $79,000 to $89,000 of modified AGI for single filers and $126,000 to $146,000 for joint filers where the contributor is covered. If only your spouse is covered, the range is $236,000 to $246,000.

Roth IRA. Same $7,000 and $8,000 limits, same April 15 deadline, and never deductible in the first place. Contribution eligibility phases out at $150,000 to $165,000 single and $236,000 to $246,000 joint.

401(k) employee deferrals. The $23,500 limit, the $7,500 catch-up at 50, and the $11,250 catch-up for people who turned 60 through 63 in 2025 all had to come out of 2025 payroll. You can't retroactively defer wages that already went out the door.

SIMPLE IRA employee deferrals. $16,500, plus $3,500 at 50 or $5,250 at 60 through 63. Same payroll rule.

HSA. $4,300 for self-only coverage, $8,550 for family, plus $1,000 if you were 55 or older. Pub 969: "You can make contributions to your HSA for 2025 through April 15, 2026." Done.

A worked example for a Schedule C filer

Let's say you run a consulting business as a single-member LLC. Net profit on Schedule C for 2025 is $150,000. You filed an extension and haven't funded anything.

The 25% SEP limit for a self-employed person is measured against net earnings after subtracting half your self-employment tax, and after the contribution itself. Pub 560 walks through the math, and it works out to about 20% of net earnings.

Self-employment tax on $150,000 is roughly $21,200. Half of that is $10,600. Net earnings for the calculation are about $139,400. Twenty percent of that is about $27,900.

At a 24% marginal rate, a $27,900 SEP contribution saves about $6,700 in federal income tax. It also goes into an account that grows tax-deferred until you take it out.

That money can go in any day between now and October 15, and it deducts on the 2025 return. Open the SEP with any brokerage, fund it, and hand the confirmation to whoever prepares your return. That's the whole process.

Two wrinkles on a solo 401(k)

If the plan already existed in 2025. The employee deferral had to be elected by December 31, 2025, but Pub 560 notes that once elected, the deposit can be made by the return due date including extensions. If you made the election in writing last year and never sent the money, you can still send it. If you never elected, that door is closed.

If you're setting the plan up now. Under the SECURE Act, a plan adopted after year end but before the extended due date of the return is treated as adopted on the last day of the prior year. That covers the employer profit-sharing piece. The employee deferral piece for a brand-new sole proprietor plan has its own rule, and that window closed on April 15. So a new solo 401(k) opened in October can still take a 2025 employer contribution, but not a 2025 deferral.

One more thing the extension didn't move

The extension gave you six months to file. It gave you zero extra days to pay. Any 2025 tax you owed was due April 15, and the failure-to-pay penalty has been running at 0.5% per month since then. Funding a SEP shrinks the bill. It doesn't pause the clock on what's left.

The short version

If you file a Schedule C or a C corp and you're on extension, you have until October 15 to fund a SEP, an employer profit-sharing contribution, or a SIMPLE employer contribution, and all three deduct on your 2025 return. IRAs, Roths, HSAs and payroll deferrals closed on April 15. S corps and partnerships had until September 15 and that's passed.

Sort your accounts into the right pile, fund what's still open, then file.

Figures verified at IRS Notice 2024-80, Publication 560 (2025), Publication 590-A (2025), and Publication 969 (2025). Every situation is different. Talk to your CPA before you move money.