Solo 401(k) Investments

How it works

A one-participant 401(k) is limited to a business with no employees other than the owner and the owner's spouse. Hiring even one employee who meets the plan's eligibility conditions ends that exemption and turns the plan into one with coverage testing and full annual filing obligations. Within that limit, contributions come in two capacities that share a single overall annual additions cap: an employee elective deferral, made pre-tax or as designated Roth, and an employer profit-sharing contribution computed off net self-employment earnings or W-2 wages. The dollar figures are indexed each year, so current IRS numbers should be checked rather than assumed. A sole proprietor computes the employer contribution from net earnings from self-employment after the deduction for one half of self-employment tax, which makes the effective contribution rate lower than the nominal percentage implies.

The plan itself is created by adopting a written plan document and obtaining a separate EIN for the trust; contributions then flow into a trust account titled in the plan's name, not the owner's. A brokerage prototype document is usually free but confines the plan to that brokerage's investment menu. A custom or checkbook-control document from a third-party administrator can open the plan to real estate, private notes, LLC interests, precious metals and private funds. Wherever the plan holds those alternatives, title sits with the trust and every expense and receipt must move through the plan's own bank or custodial account, never a personal one.

One structural distinction from an IRA matters for leveraged property: a qualified plan can meet the IRC 514(c)(9) exception for debt-financed real estate and avoid unrelated debt-financed income tax, an exception an IRA cannot use. The conditions are technical and the analysis is not a do-it-yourself exercise. Plan loans are permitted where the document allows them: up to half the vested balance, subject to a statutory dollar cap, repaid at least quarterly with interest over as long as five years, or longer for a loan used to buy a principal residence.

Once plan assets cross a filing threshold, an annual Form 5500-EZ is due, and the IRS runs a late-filer penalty relief program for one-participant plans that miss it. Throughout, the prohibited-transaction rules of IRC 4975 apply in full: no personal use of plan assets, no dealings with disqualified persons, and no compensation flowing back to the owner from a plan investment.

What it pays

The plan pays nothing on its own; the income is whatever the trust holds. Its value lies in the size of the contribution it can shelter and the breadth of what it can own, not in a rate of its own. Because an owner-only business can fund both an employee deferral and an employer profit-sharing contribution, the same savings rate builds a larger sheltered balance than an IRA allows on comparable income.

Inside the trust, interest from private notes, rent from plan-owned property and distributions from private funds all accumulate without current tax. Designated Roth deferrals carry that growth toward tax-free distribution, while the employer profit-sharing contribution stays pre-tax unless the plan document supports Roth employer contributions, an option SECURE 2.0 opened up. Where the document also allows after-tax non-Roth contributions with an in-plan conversion feature, more money can reach Roth treatment than the deferral limit alone would permit.

What ultimately decides the outcome is the underlying asset's own yield, and whether the wrapper's rules were respected along the way. A single prohibited transaction can undo the entire shelter regardless of how the asset performed.

Costs and taxes

A brokerage prototype document is often free; a third-party administrator instead charges a setup fee and an annual fee covering the document, amendments and Form 5500-EZ preparation. Plan documents must also be restated on the IRS's periodic cycle to stay qualified, a recurring cost owners routinely forget until an audit or a plan termination forces the issue.

On distribution, pre-tax money is taxed as ordinary income and qualified Roth distributions are tax-free; early distributions carry the additional 10% tax unless an exception applies. Unrelated business taxable income from an operating business held through a pass-through, and unrelated debt-financed income where the 514(c)(9) exception is not met, are taxed to the trust itself, at trust rates, on Form 990-T.

Alternative assets need an annual fair market valuation, both for plan reporting and later for required-minimum-distribution arithmetic, and the appraisal cost is a plan expense. Prohibited transactions trigger excise taxes and a correction obligation and can disqualify the plan entirely — mechanically different from the IRA version of the rule but at least as destructive. Contribution deadlines differ by capacity and track the business's tax filing deadline including extensions, and a late deposit is treated as a correction problem, not a rounding error.

Liquidity and time commitment

A plan that holds illiquid assets is itself illiquid: a private note pays on its own schedule and a property sells when it sells, not when the owner needs cash. Where the document permits, a plan loan is the main mid-life access route, but a default converts the outstanding balance into a taxable deemed distribution. Otherwise, money stays locked to the plan's distribution triggers — separation from the business, plan termination, disability, death, or the plan's stated age.

The time commitment is real, because the owner is simultaneously sponsor, trustee and administrator: deposits by deadline, annual valuations gathered, Form 5500-EZ filed once past the threshold, the document kept current through restatement cycles, and every transaction documented as it happens. Winding the plan down means formally terminating it, distributing or rolling the assets out, and filing a final return.

How it goes wrong

Hiring an employee without amending the plan converts a one-participant plan into one with coverage and testing obligations, and can disqualify it. Computing the employer contribution from gross rather than net self-employment income creates an excess contribution that must be corrected along with its earnings. Crossing the Form 5500-EZ filing threshold without noticing lets per-day penalties accrue until the late-filer relief program is used to fix it.

Buying property inside the plan and then doing the renovation personally, or letting a family member occupy it, are both prohibited transactions with a disqualified person. Paying a plan expense from personal funds, or depositing a plan asset's rent into a personal account, is the most common way checkbook-control plans fail through commingling.

Using leverage without confirming the 514(c)(9) conditions can leave the plan owing debt-financed income tax it never budgeted for. Letting the plan document go stale through a missed IRS restatement cycle puts its qualified status at risk. And treating the plan's bank account as a business account, simply because the owner happens to be signatory on both, is how the line between personal and plan assets disappears.