One-participant 401(k) plan for a business owner with no eligible common-law employees, or for the owner and a working spouse.
A solo 401(k) is a regular U.S. 401(k) plan covering a business owner with no eligible common-law employees, or the owner and a spouse who works for the business. It is also called a one-participant 401(k), individual 401(k), solo-k, or uni-k. The name describes the workforce, not a separate type of retirement account with relaxed rules.
The owner participates in two capacities: as an employee who may make elective deferrals and as the employer who may make employer contributions. That two-role structure can provide more contribution flexibility than a SEP IRA, but it also brings plan documents, deadlines, records, and possible annual filing duties.
A sole proprietor, independent contractor, partnership partner, or owner of an incorporated business may be able to sponsor a one-participant 401(k) when the business has no eligible common-law employees other than a participating spouse.
The important test is the workforce, not the business label. Calling a worker a contractor does not control if the person is a common-law employee under the applicable facts. Part-time and long-term part-time service rules can also affect eligibility. Related businesses may need to be considered together under controlled-group or affiliated-service-group rules.
A spouse can participate only when the spouse performs bona fide work for the business and has qualifying compensation. Marriage alone does not create plan compensation.
The owner can fund the plan through two contribution channels:
The total generally can be represented as:
The formula is constrained by the employee-deferral limit, overall annual-additions limit, compensation cap, catch-up rules, plan terms, and available compensation. Current IRS limits should be checked for the specific tax year.
Assume an owner receives $80,000 of qualifying W-2 compensation from the owner’s corporation. The plan permits employee deferrals and employer profit-sharing contributions. The owner elects a $12,000 employee deferral, and the corporation contributes 10% of qualifying compensation as an employer contribution. Assume all amounts are within the current limits.
| Contribution role | Amount | Main control |
|---|---|---|
| Employee elective deferral | $12,000 | Personal deferral limit, election, compensation, and plan terms |
| Employer contribution | $8,000 | Employer formula, compensation, and annual-additions limit |
| Total | $20,000 | Combined plan and tax limits |
This example illustrates the two roles; it does not state the maximum contribution. The result would change for a sole proprietor because adjusted net earnings replace W-2 compensation.
A sole proprietor or partner generally calculates plan compensation from net earnings from self-employment after adjustments, including the deductible part of self-employment tax and the owner’s own employer contribution. That circular relationship requires the rate table or worksheets in IRS Publication 560.
In simplified form:
Gross receipts, owner draws, or S corporation shareholder distributions are not automatically plan compensation. An S corporation shareholder-employee generally needs qualifying W-2 compensation to support contributions.
An owner can have a day job and a separate business with a solo 401(k), but the employee elective-deferral limit generally applies to the person across plans. If the person already deferred part of the annual limit into another employer’s plan, only the remaining amount may be available for employee deferral to the solo plan.
Employer contributions are analyzed separately, and related-employer rules can change the combined limits. The existence of two plan accounts does not create two independent personal deferral limits.
Suppose a person makes $10,000 of elective deferrals to an unrelated employer’s 401(k) and later considers a deferral to a solo 401(k). The $10,000 already used must be counted against that person’s annual elective-deferral ceiling. The solo plan provider may not know about the other plan, so the owner must coordinate the records.
| Feature | Solo 401(k) | SEP IRA | Traditional or Roth IRA |
|---|---|---|---|
| Sponsor | Owner’s business | Employer or self-employed business | Individual |
| Main contribution source | Employee deferral plus employer contribution | Employer contribution | Individual contribution |
| Common-law employees | Generally not compatible once employees become eligible | Can cover employees under the SEP formula | Not an employer plan |
| Employee salary deferrals | Yes | No for a regular SEP | Not payroll deferral under an employer plan |
| Roth feature | May be available if the plan permits | May be available under current rules if supported | Roth IRA is a separate IRA type |
| Participant loan | May be available if the plan permits | Not permitted | Not permitted |
| Annual plan filing | Can become required | Generally no Form 5500 | No employer plan filing |
| Administrative burden | Higher | Lower | Usually lowest |
A solo 401(k) may allow useful contributions at a lower level of business income because employee deferrals are separate from the employer percentage. A SEP can be easier to establish and maintain. A personal IRA can supplement either arrangement but has a separate contribution framework. None is universally best.
The absence of common-law employees is what removes normal employee nondiscrimination testing from a one-participant plan. When the business hires employees who satisfy the plan’s eligibility rules, the plan may need to cover them, perform applicable testing, provide notices, and meet broader reporting and fiduciary requirements.
An owner should not wait until year-end to discover that a worker became eligible. Review dates of hire, hours or service periods, age, excluded classifications, related employers, and plan-entry dates during the year. Excluding an eligible employee is an operational failure that may require correction.
A solo 401(k) requires a written plan document and trust or custodial arrangement. The business should establish the plan, make deferral elections, and deposit contributions by the deadlines applicable to the entity, participant, contribution type, and tax year.
The deadline for adopting a plan is not necessarily the deadline for making an employee elective-deferral election. Employer contributions can have a different funding deadline. Retroactive or late actions should not be assumed valid merely because a tax return is on extension.
Keep:
The IRS generally requires Form 5500-EZ when a one-participant plan reaches the applicable year-end asset threshold. A final return can also be required when the plan is terminated, even if assets are below the normal threshold. Current form instructions determine the filing obligation and deadline.
Missing a filing because the provider did not send a reminder does not remove the plan sponsor’s responsibility. The owner should track total plan assets across relevant one-participant plans and document when all assets have been distributed at termination.
The plan must also operate according to its written terms. Roth accounts, loans, hardship distributions, rollovers, and investment options exist only when the document and provider support them and the legal requirements are met.
A solo 401(k) is an account structure, not an investment. Its risk and return depend on the assets selected. Provider menus range from limited fund lists to broader brokerage access, and fees can include setup, annual administration, custody, transaction, fund, loan, and termination charges.
Distributions generally require a plan-permitted event and follow 401(k) tax rules. A plan loan may be available if the document permits it, but borrowing creates repayment, default, employment-status, and cash-flow risks. A loan is not equivalent to a tax-free withdrawal.
The IRS one-participant 401(k) guide explains the owner employee-employer roles, cross-plan deferral coordination, employee testing, and Form 5500-EZ threshold. IRS Publication 560 provides self-employed plan calculations. The Department of Labor small-business plan guide compares employer retirement structures and responsibilities.
This article provides general financial education, not tax, legal, payroll, fiduciary, retirement-plan, or investment advice. Eligibility, limits, deductions, deadlines, and filings depend on current law, the written plan, entity, compensation, workforce, related employers, other plans, provider, and tax year.