Can I Have a Solo 401k and a Regular 401k? Caps and Deadlines

Yes, you can have a solo 401k and a regular 401k at the same time, as long as you have W-2 wages from an employer and separate net earnings from a self-employment activity. The catch is that your personal employee deferrals are capped across both plans combined at $24,500 for 2026, while the employer-side contribution limit applies separately to each plan. That second half is where the strategy earns its keep.¹

What You Need to Qualify

Two income streams, from two different sources. The regular 401k comes through your day job on a W-2. The solo 401k requires income from a legitimate trade or business you operate for profit, typically reported on Schedule C or through 1099-NEC contract work.²

The solo 401k also has a headcount rule: your side business can have no full-time employees other than you and your spouse. “Full-time” for this purpose means anyone working more than 1,000 hours in a year, so occasional contractors or truly part-time helpers below that line don’t disqualify you.³

Starting in 2026, watch a second threshold created by the SECURE 2.0 Act. If a part-time worker logs at least 500 hours in each of two consecutive plan years, they become eligible to make elective deferrals under your plan.⁴ Once that happens, the plan stops qualifying as a one-participant plan. If you regularly use the same part-time assistant, track their hours.

The Shared Employee Deferral Cap

The amount you personally defer as an employee is capped across every 401k plan you participate in, combined. For 2026 that ceiling is $24,500.¹ The limit follows you, not the plan. Defer $18,000 into your employer’s 401k and you have only $6,500 left for the employee side of your solo 401k.

Your employer’s plan and your solo 401k don’t talk to each other. Nobody is going to stop you from over-contributing in real time; the responsibility is yours. A spreadsheet tracking every payroll deferral across both plans, updated monthly, is the reliable way to stay within the line.

Catch-Ups for Ages 50 and Up

If you turn 50 or older during 2026, you can defer an additional $8,000 on top of the base, bringing your personal deferral capacity to $32,500. This catch-up is also shared between the two plans.¹

SECURE 2.0 added a larger catch-up for participants who turn 60, 61, 62, or 63 during the plan year. For 2026, this amount is $11,250 and replaces the standard $8,000 rather than stacking on top, pushing total personal deferral capacity to $35,750 for those in that narrow window.¹ Once you turn 64, you drop back to the regular $8,000 catch-up.

Where the Dual-Plan Strategy Actually Pays

The employee deferral limit is shared. Employer-side contributions are not. That single fact is the entire reason for running both plans.

Each employer’s plan has its own Section 415(c) ceiling, which for 2026 is $72,000 (or $80,000 with the standard catch-up, $83,250 with the age-60-to-63 catch-up).⁴ That $72,000 cap applies separately to your W-2 employer’s plan and to your solo 401k.

In practice: your W-2 employer might contribute a match or profit-sharing amount on your behalf. None of that reduces the room available in your solo 401k. On the solo side, you make employer profit-sharing contributions of up to 25% of your compensation from the business. Combined with your employee deferral, the solo 401k total from all sources still can’t exceed $72,000 before catch-up, but that ceiling is yours alone.³

How Business Structure Changes Your Contribution Math

How you calculate the employer-side contribution on the solo 401k depends on how the business is organized, and the difference directly controls how much you can put in.

Sole Proprietorships and Single-Member LLCs

If your business is unincorporated, your compensation for contribution purposes is net self-employment income after two deductions: half of your self-employment tax and the contribution itself. Because the contribution depends on the income and the income depends on the contribution, you use a reduced rate. A plan with a stated 25% contribution rate works out to roughly 20% of net self-employment earnings after the adjustments. IRS Publication 560 provides the rate tables and worksheets.⁵

S Corporations

If you run the business through an S corporation, employer contributions are based on 25% of the W-2 wages the corporation pays you. S corporation distributions and dividends do not count as compensation for retirement plan purposes.⁶ Setting your salary low to trim payroll taxes can quietly shrink your allowable 401k contributions.

The Controlled Group Trap

One boundary catches people off guard. If you own 80% or more of two businesses, the IRS treats them as a single employer for retirement plan purposes. Employees from both businesses must be aggregated when testing plan eligibility, so you cannot set up a second company with no employees, open a solo 401k through it, and exclude the employees from your first business.⁷

Aggregation also applies to “brother-sister” controlled groups, where the same five or fewer owners hold at least 80% of each business and share at least 50% identical ownership. It extends to affiliated service groups, where a service organization and related entities regularly perform services together. For the Section 415 contribution limits specifically, the ownership threshold drops to more than 50%.

If your side business and your W-2 employer are related through common ownership, get professional advice before opening a solo 401k. A disqualified plan carries severe tax consequences.

Setup and Funding Deadlines

To make contributions for a given tax year, your solo 401k must be formally established by December 31 of that year. The plan documents need to be signed and adopted by that date, even if you don’t deposit money until later. Miss December 31 and you cannot retroactively create the plan for the year that just ended.

Once the plan exists, employee salary deferrals for a given year must be elected by December 31 of that year. Employer profit-sharing contributions can be deposited any time up to the business’s tax filing deadline, including extensions. For a sole proprietor filing on a calendar year, that means as late as October 15 with an extension.³

If You Go Over the Deferral Limit

Because no one is tracking the combined total for you, this is the most common and most expensive mistake in a dual-plan setup. If you accidentally exceed the $24,500 base (or the applicable catch-up ceiling), you must pull the excess out by April 15 of the following year. A timely correction means the excess gets taxed in the year you deferred it, and the earnings on the excess are taxed in the year they’re distributed. No additional penalties.⁸

Miss April 15 and the math turns painful. The excess is taxed twice, once in the contribution year and again when eventually distributed. Late distributions can also trigger a 10% early distribution tax, 20% mandatory withholding, and spousal consent requirements.⁸ The moment you notice you’ve gone over, act before the calendar closes on you.

1
Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
2
Internal Revenue Service. Self-Employed Individuals – Calculating Your Own Retirement Plan Contribution and Deduction
3
Internal Revenue Service. Retirement Plans for Self-Employed People
4
Internal Revenue Service. Notice 2025-67 – 2026 Amounts Relating to Retirement Plans and IRAs
5
Internal Revenue Service. Publication 560 (2025), Retirement Plans for Small Business
6
Internal Revenue Service. One-Participant 401(k) Plans
7
Office of the Law Revision Counsel. 26 U.S. Code 414 – Definitions and Special Rules
8
Internal Revenue Service. 401(k) Plan Fix-It Guide – Elective Deferrals Werent Limited to the Amounts Under IRC Section 402(g)