How to Open a Solo 401(k) in 2026: A Step-by-Step Guide

A solo 401(k) lets business owners contribute up to $72,000 in 2026, plus catch-up contributions of $8,000 (or $11,250 if you're 60-63). S Corp owners just need to coordinate W-2 wages with contributions. Lettuce pairs your Solo 401(k) with your payroll, making it simple to optimize your tax savings.

A solo 401(k), also called a one-participant 401(k), lets an eligible business owner contribute in two roles: as an employee and as the employer. It is not a separate type of retirement plan. It is a 401(k) plan designed for an owner-only business, which means the setup documents, contribution rules, and filing requirements still matter. Opening one requires six steps and is manageable when you work in the correct order:

1. Confirm Eligibility
2. Choose a Provider
3. Adopt the Written Plan
4. Coordinate the Deadlines (with your plan and payroll)
5. Calculate both Contribution Layers
6. Keep the Plan Compliant (after it is funded)

The steps cover eligibility, provider selection, plan documents, deadlines, and contribution strategy. See how Lettuce connects your Solo 401(k) contributions to your full tax and payroll picture.

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Why A Solo 401(k) Can Be Worth the Extra Setup

The advantage is the two-part contribution structure. As the employee, you can make an elective deferral. As the employer, your business can make a non-elective contribution. That combination can allow you to reach the annual limit at a lower income than with a SEP IRA, even though both arrangements are generally subject to the same overall defined-contribution ceiling. A Solo 401(k) can also permit catch-up contributions, which a SEP IRA does not.

Tax treatment depends on the contribution source. Traditional elective deferrals generally reduce current federal taxable income, but W-2 deferrals remain subject to Social Security and Medicare taxes. Roth deferrals do not create a current income-tax deduction, but qualified distributions can be tax-free. Employer contributions and optional after-tax contributions follow their own rules, so compare a plan's features before you choose a provider.

Step 1: Confirm That You Are Eligible to Open a Solo 401(k) Before You Do Anything Else

The IRS describes a one-participant 401(k) as a plan covering a business owner with no common-law employees, or the owner and spouse. Certain partner-only businesses may also qualify. The important point is not whether someone is casually described as full-time or part-time; it is whether the business has employees who must be eligible under the plan and tax rules.

Before selecting a provider or developing a contribution strategy, review three areas: business structure, payroll and compensation, and employee eligibility. These factors can affect both plan qualification and the calculation of allowable contributions. For example, a sole proprietor and an S corporation shareholder may both be eligible for a solo 401(k), but their solo 401(k) contribution calculations differ because the compensation used for retirement plan purposes is determined differently.

Confirming eligibility at the outset helps ensure the appropriate retirement plan is established and reduces the risk of compliance issues later. It also provides a sound basis for comparing a solo 401(k) with alternatives such as a SEP IRA and determining which option best supports the owner's retirement and tax-planning objectives. If you’re not sure, it may be worth speaking with a tax professional.

Step 2: Choose A Solo 401(k) Provider Based on the Plan

Picking a solo 401(k) provider is all about finding one that fits how your business actually runs. Here is what actually separates a good fit from a frustrating one.

• All-in Fees: Review the annual plan fee, asset-based advisory fee, brokerage and custody charges, alternative-asset fees, loan or distribution fees, rollover fees, and any Form 5500 preparation charge.
• Contribution Options: Check for traditional and Roth elective deferrals, employer contributions, and - if relevant - voluntary after-tax contributions and in-plan Roth conversions. A 'mega backdoor Roth' is not available unless the document and recordkeeping system support the required features.
• Rollover support: Ask what incoming assets the plan accepts. A provider is not required to accept rollovers, and not every IRA or Roth balance is eligible to move into a qualified plan. Compare available investments, cash handling, self-directed options, transaction limits, and prohibited-transaction support. More investment flexibility also creates more compliance responsibility.
• Compliance support: Once your plan assets exceed $250,000, the IRS requires an annual Form 5500-EZ filing. The DOL recommends documenting your provider selection process, so choose one that tracks this threshold and handles the filing without adding it to your to-do list.
• Payroll and Tax Coordination: For S corporation owners, retirement plan contributions must be coordinated with W-2 compensation and payroll. Selecting a provider or platform that integrates with the company's payroll process can help improve contribution accuracy and reduce year-end adjustments. Lettuce currently partners with Carry to help S corporation owners coordinate payroll and retirement contributions.

Before selecting a provider, consider using a solo 401(k) calculator to estimate your potential contribution based on your business structure and eligible compensation. Understanding your contribution limits in advance can help you select a provider that supports your retirement objectives, tax-planning strategy, and administrative needs.

Step 3: Adopt the Plan and Open the Correct Accounts

A brokerage or bank application is not a substitute for a written 401(k) plan. The IRS identifies four basic setup actions: adopt a written plan, arrange a trust for plan assets, create a recordkeeping system, and provide required plan information to participants. In an owner-only plan, one person may fill several (or all) roles, but the records still need to be complete.

Precision at this stage can help protect your deductions and prevent potentially costly mistakes down the road. Going forward, a complete setup should include:

• Documentation of the business’s legal name, entity type, tax year, address, and EIN
• A signed plan and adoption agreement, including the effective date and selected plan features
• A designated plan administrator and trustee
• A plan, trust, or custodial account
• Participant and beneficiary information for the owner and participating spouse, if applicable
• A written elective-deferral election and payroll instructions showing when contributions will be withheld from W-2 compensation
• A recordkeeping system that separately tracks employee deferrals, employer contributions, Roth contributions (if applicable), voluntary after-tax contributions, rollovers, earnings, fees, and distributions

Plans with assets over $250,000 also trigger an annual Form 5500-EZ filing. The solo 401(k) deadline determines which tax year your contributions count toward, making early document completion part of the strategy.

IMPORTANT: Keep the signed plan documents, amendments, elections, account statements, contribution calculations, payroll reports, and tax filings together. Operating the plan differently from its written terms can create a qualification failure even when the total dollars look right.

Step 4: Get your Solo 401(k) Deadlines on the right calendar

The plan-adoption deadline, the employee election deadline, the deposit date, and the employer-contribution deadline are related but not interchangeable. Provider processing cutoffs may also be earlier than the legal deadline, so do not wait until the last week of the year.

The timeline below maps out each solo 401(k) deadline and milestone so you can get an estimate of where you land.

• Owner-employees are generally elected by the end of the tax year. IRS Publication 560 states that owner-employee deferrals must be elected by year-end and can then be contributed by the tax-return filing deadline, including extensions. The plan document or provider may require earlier action.
• S Corp deferrals must be coordinated with payroll. An S Corp owner cannot retroactively turn shareholder distributions or wages that were already available into an elective deferral. Put the election in place before the relevant compensation is paid, process the contribution through payroll, and make sure Form W-2 reports it correctly.
• A first-year sole proprietor has a narrow exception. A sole proprietor with no employees who is the only participant may adopt a new 401(k) after year-end and make first-plan-year elective deferrals by the tax-return filing deadline determined without extensions. Confirm that the facts and provider documents fit the special rule before relying on it.
• Your S Corp payroll calendar and your retirement calendar are connected. Salary decisions made in January affect how much room you have to contribute before year-end deadlines close.
• Model your contributions before the window closes. Use a solo 401(k) calculator to see how much you can put away based on your income and entity type, so you are not guessing when the deadline is weeks away.

Step 5: Calculate Solo 401(k) Contribution Limits In the Correct Order

The value of a solo 401(k) comes from the ability to contribute in two capacities: as an employee and as the employer. For 2026, the employee elective deferral limit is $24,500, while employer contributions may provide additional contribution capacity based on eligible compensation. The combined annual contribution limit is generally $72,000, excluding applicable catch-up contributions. Participants age 50 or older may contribute an additional $8,000 in 2026, while participants ages 60 through 63 may qualify for the higher $11,250 catch-up contribution.

Understanding the annual limits is only part of the calculation. The employer contribution is determined differently depending on how the business is structured. For an S corporation owner, employer contributions are generally based on eligible W-2 compensation. For a sole proprietor, the calculation is based on adjusted net earnings from self-employment, taking into account the applicable self-employment tax adjustment and retirement plan contribution rules.

While maximizing contributions can provide significant retirement and tax-planning benefits, the appropriate contribution should also reflect the business owner's compensation, taxable income, and available cash flow. Calculating the contribution before funding the plan can help determine an appropriate amount for the year. S corporation owners should coordinate retirement planning with payroll throughout the year because W-2 compensation directly affects employer contribution capacity.

Step 6: Fund, reconcile, and maintain the plan

A contribution is complete only when the amount is deposited, assigned to the correct source, and reflected consistently in payroll, plan records, and tax filings. Use a short closeout process each year.

• Finalize the contribution calculation. Coordinate eligible compensation, contributions to other plans, plan features, cash flow, and the entity-specific employer formula.
• Process employee deferrals correctly. Keep the signed election, use the right payroll code, and deposit the amount under the plan's timing procedures.
• Label every contribution source. Traditional deferral, Roth deferral, employer contribution, after-tax contribution, and rollover money must be tracked separately.
• Make the employer contribution by the applicable deadline. Fund it from the business and retain the calculation and proof of deposit.
• Reconcile the records. Match payroll reports, Form W-2, provider statements, general-ledger entries, and the business and personal tax returns
• Monitor plan status. Recheck employee eligibility, related businesses, plan amendments, beneficiary designations, and year-end asset value every year.
• File when required. A one-participant plan generally files Form 5500-EZ or, when eligible, Form 5500-SF after year-end assets cross the $250,000 threshold. A final return is required when the plan terminates, and all assets are distributed.

How to Open a Solo 401(k): Frequently Asked Questions (FAQs)

Establishing a solo 401(k) correctly is only the beginning. Business owners should also understand how rollovers, contribution limits, reporting requirements, and compensation rules can affect the plan over time. The following questions address several of the most common issues that arise when managing a solo 401(k).

Can I roll over an old 401(k) or IRA into my solo 401(k)?

Often, but the receiving plan must allow it, and the source must be eligible. A direct rollover or trustee-to-trustee transfer normally avoids mandatory withholding. Traditional pre-tax IRA and eligible employer-plan assets can often move into a qualified plan; a Roth IRA cannot be rolled into a 401(k). After-tax amounts, inherited accounts, required minimum distributions, and other special balances require additional review.

Does a solo 401(k) plan require any IRS filings?

A one-participant plan is generally exempt while combined year-end plan assets are $250,000 or less. Once assets cross the filing threshold, Form 5500-EZ or an eligible Form 5500-SF is generally due by the last day of the seventh month after the plan year ends. A final return is required after termination when all assets have been distributed, even if the balance was below the threshold before distribution.

How should an S Corp owner coordinate W-2 wages with solo 401(k) contributions?

Start with a defensible, reasonable-compensation analysis, not a desired retirement number. Then model the employee deferral and employer contribution using W-2 wages, sign the election before the relevant payroll, process deferrals through payroll, and reconcile the W-2 and employer contribution before the business return is filed. Shareholder distributions never substitute for W-2 plan compensation.

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Open the Plan With a System That Keeps Payroll & Tax Records Aligned

The account-opening form is only the first milestone. The real value comes from coordinating the plan document, compensation, payroll, contribution sources, deposits, and tax filings throughout the year. When those records agree, you are less likely to miss a deadline, overfund the plan, or discover a reporting problem after the return is filed.

Lettuce and Carry currently have a partnership that benefits S Corp owners. Through Lettuce, eligible owners can review Carry's solo 401(k) offering alongside the W-2 salary and payroll workflow used for the business. Features, pricing, eligibility, and investment options can change, so review the current plan materials before enrolling and confirm the contribution amount with a qualified tax professional.

Review the current Carry Solo 401(k) Lettuce offering.

The Lettuce Learning Center is operated by Lettuce Financial Labs Inc. (“Lettuce”) and contains generalized educational content about personal finance topics. The information published should not be construed as personalized investment or tax advice and should not be considered as a solicitation to buy or sell any security or engage in a particular investment, accounting, tax or legal strategy. You should consult with qualified tax, legal, accounting, and investment professionals regarding your specific situation.

The accounts, strategies and/or investments discussed in this material may not be suitable for all. All investments involve the risk of loss, and past performance does not guarantee future results. Investment growth or profit is never a guarantee. All statements and opinions included on the Lettuce Learning Center are intended to be current as of the date of publication but are subject to change without notice.

Carry is owned by Lettuce.; the two are affiliated. Any investment advisory services discussed on this website are provided exclusively through its wholly-owned subsidiary, Carry Advisors LLC (“Carry Advisors”), an SEC registered investment adviser.

To access Carry investment advisory services through Carry Advisors, you must be a client of Carry on an eligible membership plan. For more information about Carry Advisors’ investment advisory services, please see our Form ADV Part 2A brochure and Form CRS or through the SEC’s website at www.adviserinfo.sec.gov.