Who qualifies for a solo 401(k)
The IRS sets two uncompromising eligibility requirements for a solo 401k for self employed individuals: you must generate self-employment income, and you must have no full-time employees.
Your business structure determines how your income qualifies. Sole proprietors, single-member LLCs, S-Corporation owner-employees, and general partners in a partnership all fit the bill. The income must be earned self-employment income—not passive earnings from investments, real estate rentals, or stock dividends.
The employee rule is where most business owners trip up. You cannot sponsor a Solo 401(k) if you employ anyone who works more than 1,000 hours during the plan year. Part-time workers under that threshold are fine. Independent contractors working on 1099 contracts do not count as employees either.
There is one major exception to the single-worker rule: your spouse. If your spouse earns income from your business, they can join the plan as a second participant. That effectively doubles your household contribution limit.
Among small business retirement plans, this total contribution cap makes the Solo 401(k) uniquely aggressive for lean businesses.
Prototype brokers vs. custom self-directed plans
Before opening an account, you must pick the legal framework that governs your retirement trust. Providers split into two distinct categories, each serving a completely different investment strategy.
| Feature | Free Prototype Plans (Schwab, Fidelity) | Self-Directed Custom Plans |
|---|---|---|
| Setup & Maintenance Fees | $0 setup, $0 annual fee | $300–$1,000 setup, $100–$300 annual fee |
| Allowed Investments | Stocks, ETFs, mutual funds, bonds | Real estate, crypto, precious metals, private equity |
| Participant Loans | Rarely allowed | Permitted (up to $50,000 or 50% of balance) |
| Roth Option | Available at select brokers | Almost always built into the plan |
| Checkbook Control | No | Yes (via dedicated bank account) |
Standard prototype plans work best if you plan to stick to publicly traded securities. Major brokerages provide pre-approved IRS document packages for free. You open the account online, transfer money, and pick your index funds.
Custom, self-directed plan documents cost real money up front, but they grant total asset flexibility. With a self-directed setup, your plan acts as an independent entity with its own dedicated bank account under your signature control.
This structure offers a major tax edge over a self directed IRA. If a self-directed IRA buys real estate using a non-recourse mortgage, the profits generated by that leverage incur Unrelated Debt-Financed Income tax (UDFI). A Solo 401(k) is exempt from UDFI on acquisition indebtedness for real estate. That distinction saves real estate investors thousands in unexpected taxes.
Calculating employee and employer contributions
The core advantage of the account lies in its two-tiered structure. You act as both the worker receiving a paycheck and the owner distributing profits.
The employee elective deferral lets you contribute 100% of your earned income up to $23,000 per year (or $30,500 if aged 50 or older). This portion can be traditional pre-tax or Roth, provided your plan document permits Roth contributions.
The employer profit-sharing contribution lets your business add extra funds on top of your employee deferral. The math here depends on your corporate entity:
- S-Corporations or C-Corporations: You can contribute up to 25% of the W-2 salary you pay yourself.
- Sole Proprietorships and Single-Member LLCs: You can contribute up to 20% of net self-employment earnings (calculated as net Schedule C profit minus half of your self-employment tax).
Total contributions across both sides cannot exceed $69,000 per year per participant, excluding age-based catch-ups.
Pay attention to the calendar. You must formally adopt your plan document by December 31 of the tax year for which you want to make employee deferrals. You then have until your business tax filing deadline (including extensions) to actually deposit the profit-sharing funds.
The step-by-step setup process
Setting up your plan takes about an hour of paperwork if you gather your tax documents beforehand. Follow these five steps to ensure IRS compliance.
1. Obtain a dedicated EIN for the trust
Your plan is a distinct legal trust. Do not use your personal Social Security Number or your primary business EIN to hold account assets. Visit the IRS website and apply for a new Employer Identification Number specifically categorized as a “Trust” or “Retirement Plan.”
2. Execute the adoption agreement and plan documents
If you choose a mainstream brokerage, you fill out their online application. If you choose a self-directed provider, they draft an IRS-approved prototype plan. Read every line of the Adoption Agreement. This document defines your plan rules, including whether you allow Roth contributions, participant loans, or voluntary after-tax contributions. Sign and date it before midnight on December 31.
3. Open the trust bank or brokerage accounts
Take your executed plan documents and plan EIN to your chosen broker or bank. Open an account titled in the name of your trust—for example, The Oak Street Consulting 401(k) Trust. If your spouse participates, open a separate sub-account under the same trust structure to track their assets individually.
4. Record a written election form
Even if you run a solo business, document your intent. Write a formal salary deferral election memo detailing how much income you plan to defer into the account for the tax year. Keep this document in your permanent corporate records. The IRS wants proof that you elected your deferrals before earning the money.
5. Fund the account properly
Transfer funds directly from your business checking account into the trust account. Never deposit personal funds directly into a traditional Solo 401(k) without routing them properly through your business bookkeeping. Clear paper trails keep audits painless.
Ongoing administration requirements
Solo 401(k) plans carry minimal administrative overhead compared to traditional corporate plans, but they are not entirely maintenance-free.
The most critical administrative hurdle is IRS Form 5500-EZ. You do not need to file this form when starting out. However, once the total assets across all your Solo 401(k) accounts exceed $250,000 at the end of any plan year, filing Form 5500-EZ becomes mandatory.
Filing is due by the last day of the seventh month following the end of the plan year—July 31 for calendar-year plans. Missing this deadline triggers hefty penalties from the IRS, running $250 per day up to a maximum of $150,000.
Keep your plan documents updated. When tax law changes, your plan provider will issue plan restatements or amendments. Sign these promptly when prompted by your document provider to keep your trust in compliance.
FAQ
Can I contribute to a Solo 401(k) if I already have a 401(k) through a full-time W-2 job?
Yes, but your employee deferral limit ($23,000) applies across all 401(k) plans combined across all employers in a single calendar year. However, your employer profit-sharing limit is calculated independently for your self-employed business, capped at 25% of W-2 pay or 20% of net self-employment earnings up to the overall ceiling.
What happens if I hire a full-time employee later?
If you hire an employee who works more than 1,000 hours in a 12-month period, your plan loses its solo status. You must convert the plan into a standard full-scale 401(k), offering coverage and non-discrimination testing to that employee, or close the plan and roll the assets into an IRA.
Can I take a loan from my Solo 401(k)?
Only if your plan document explicitly allows participant loans. Free brokerage prototype plans usually prohibit loans, whereas custom self-directed plans typically permit borrowing up to 50% of your vested balance or $50,000, whichever is less, repaid over five years with market-rate interest paid back to yourself.
Is a Solo 401(k) better than a SEP IRA?
It depends on your income. A Solo 401(k) allows you to hit high contribution levels at lower income ranges because you can stack an employee deferral on top of profit sharing. A SEP IRA relies entirely on employer profit-sharing percentages, requiring higher gross earnings to reach the same tax-deferred amount.
What to do next
Opening a Solo 401(k) gives self-employed business owners maximum control over their tax liability and retirement savings velocity. Start by selecting your plan provider before the fourth quarter ends, request your trust EIN, and sign your adoption agreement before December 31 to secure your tax deductions for the current year.
This guide provides general educational information and should not be construed as personal tax, legal, or financial advice; consult a qualified Certified Public Accountant (CPA) or fee-only financial advisor to evaluate your specific tax strategy before opening or funding a retirement plan.