How To Set Up A Solo 401(k) Step By Step Before The Tax Deadline

Open the account by the tax deadline (usually April 15) by completing an adoption agreement with a provider that allows same-day online establishment, then fund the employee deferral portion and file Form 5500-EZ if assets exceed $250,000.

1. The provider paperwork for your solo 401k setup

Your first step is to obtain an EIN from the IRS website. It takes five minutes and is free. Every solo 401(k) provider requires it on the adoption agreement. Using your SSN is the #1 reason for instant rejection. Next, log into your chosen provider. Options include Fidelity, Vanguard, Schwab, or a DIY platform like E*Trade or Interactive Brokers. Select “Solo 401(k)” or “Individual 401(k)” from the retirement account menu. You will complete the adoption agreement. This two-page document asks for your name, EIN, plan effective date, and your election for staff salary reductions. Use January 1 of the tax year you’re funding as the effective date, even if you’re filing in April. The most common rejection reason is entering the wrong plan year. If you’re filing 2024 taxes, the plan year must be 2024, not 2025. Another frequent error is forgetting to sign the “spousal consent” section if you live in a community property state. After submission, you’ll receive a confirmation number and a plan document PDF. Save both, because the IRS can audit your establishment date. Do not fund the account until the provider confirms the plan is active. This is usually instant but can take one business day for manual review.

2. Funding the worker vs. employer side correctly

Once the plan is live, you must split your deposit into two buckets. The staff salary reduction is due by the tax deadline. For the 2024 tax year, the limit is set by the IRS at $23,000, or $30,000 if you’re 50 or older. Visit IRS.gov for the current year’s limit. The deadline is April 15, 2025. The employer profit-sharing portion is due later. For 2024, the IRS caps this combined amount at $46,000. Check the official IRS tables for the precise figure. The deadline is your extended filing deadline, typically October 15, 2025. If you mix them up by contributing employer money before the staff salary reduction, you’ll over-contribute to the worker side if you later try to max it. This causes a 6% excise tax on the excess. The excess must be withdrawn by April 15 of the following year. The correct order is crucial. First, deposit the staff salary reduction amount and write “2024 employee deferral” in the memo line. Then, after the tax deadline, calculate and deposit the employer allocation. If you file for an extension using Form 4868, you automatically extend the employer allocation deadline but not the staff salary reduction deadline. You can fund the profit-sharing portion as late as October 15. The salary reduction must be in by April 15. Track both deposits separately in your records. Your provider will send a Form 5498 in May showing the salary reduction. A separate Form 5498 for the employer side arrives in January.

3. When you cannot make the deadline

If you miss the April 15 cutoff for the staff salary reduction, that portion of your deduction is gone permanently. You cannot make it up later. Any deposit made after the deadline is treated as a 2025 contribution, reducing next year’s limit. The one exception is the employer profit-sharing allocation. If you filed Form 4868 by April 15, you have until October 15 to make that deposit and still deduct it on your 2024 return. But if you miss the extended deadline too, you lose the entire deduction for the year. You must withdraw the mistaken deposit plus earnings by the applicable deadline to avoid the 6% excise tax. There is no grace period. No “reasonable cause” extension exists for a solo 401(k) establishment. The plan must exist by December 31 of the tax year. Alternatively, it must exist by April 15 of the following year if you use the “deemed established” rule, but only if you make a contribution by the tax deadline. If you miss it, your only option is to open a SEP-IRA instead. A SEP-IRA allows deposits up to 25% of net earnings but with a lower cap. This is a common fallback. You should compare it against the solo 401(k) vs sep-IRA tradeoffs before switching.

4. The forgotten form 5500-ez trap

Once your solo 401(k) balance crosses the threshold on December 31 of any year, you must file Form 5500-EZ by July 31 of the following year. The IRS sets this filing trigger at more than $250,000. Confirm the current threshold on the official IRS Form 5500-EZ instructions page. You must file even if you’re still working and making deposits. The penalty for filing late is $250 per day, up to a maximum the IRS sets at $150,000. The IRS has been aggressively enforcing this with automatic letters. To check if you owe a filing, log into your provider’s portal and look at your December 31 statement from the prior year. If the total exceeds the IRS threshold, you must file. If you already missed the deadline, file immediately using the IRS’s late-filer penalty relief program before the IRS contacts you. This reduces the penalty to a flat fee set by the program at $750 for the first year. The IRS publishes this fee on its DFVCP page. This reduction only applies if you file before receiving a notice. Most self-employed people forget this form because they assume it’s only for large plans. The trigger is the IRS threshold, not $1 million. If you’re unsure, err on the side of filing. A zero-balance Form 5500-EZ is cheap insurance against a five-figure penalty.

5. Frequently Asked Questions

Can I open a solo 401(k) after December 31 but before April 15?

Yes, but only if you make a contribution by the tax deadline. The plan is considered “established” on the date you sign the adoption agreement. You have until April 15 to make the staff salary reduction. The plan documents must be signed by December 31 of the tax year to count for that year. The exception is if you use the deemed establishment rule, but this requires that you actually fund it by April 15.

What if I already have a SEP-IRA from a previous year, can I still open a solo 401(k)?

Yes, but you cannot roll the SEP-IRA funds into the solo 401(k) if the SEP contains employer contributions from a prior year. Only staff salary reductions or rollovers from other 401(k)s can move in. You can keep the SEP separate and make solo 401(k) contributions on top. You must stop SEP contributions for the current year to avoid exceeding the combined contribution limit. Compare the two plans side by side before switching, because the sep-IRA vs simple IRA rules differ from solo 401(k) rules.

Do I need to file a Form 5500-EZ if my balance is exactly the threshold?

No, the IRS rule is “more than $250,000.” An exact balance at the threshold does not trigger the filing requirement. Check the official IRS Form 5500-EZ instructions for the precise language. If your balance crosses that amount on December 31, you must file by July 31 of the next year. If you’re uncertain, check your December 31 statement. If it shows a single dollar over the limit, you owe the form.

Open the account by the tax deadline, usually April 15. Complete an adoption agreement with a provider that allows same-day online establishment. Then fund the staff salary reduction portion. File Form 5500-EZ if assets exceed the IRS-published threshold, currently $250,000. You must have a valid EIN, not your Social Security number, before you sign anything. The account must be legally “established” by midnight local time on the deadline, not just funded. The entire process takes about 20 minutes of paperwork. This is only true if you know the exact order and avoid the common rejections that push you past the cutoff. For a deeper dive into the broader topic of self-employed retirement plans, see Self-Employed Retirement Plans: What to Know and How to Handle It.