Solo 401(k) and Employer 401(k): Deferral Cap and 415 Limits

You can contribute to a Solo 401(k) and an employer 401(k) in the same year. The catch is that one limit is shared between them and one is not: the employee elective deferral cap of $24,500 for 2026 applies to you as a person across every 401(k) you touch, while the employer profit-sharing contribution from your side business is calculated on its own and does not shrink because of anything your day job puts in.¹ Get that distinction right and the dual-plan setup lets you save well past what either plan could hold alone.

Who Can Use Both Plans

A Solo 401(k) is open to any business owner whose only employees are themselves and, optionally, a spouse.² Freelance work, a consulting LLC, or any other venture that produces self-employment income qualifies. Holding a W-2 job with a traditional 401(k) at the same time does not disqualify you.

A spouse who works in the business can participate in the Solo 401(k) too, either as a W-2 employee or as a co-owner. Each spouse gets their own deferral limit and their own employer contribution, which effectively doubles the household’s capacity through the side business.

The plan loses its solo status the moment the business hires anyone other than a spouse who works more than 1,000 hours per year. Once that happens, eligible employees must be included and the plan has to meet nondiscrimination testing, which is a much heavier administrative lift.²

The Deferral Limit Is One Number Across Both Plans

The employee deferral cap under 26 U.S.C. § 402(g) is a per-person limit.³ For 2026, $24,500 is the combined total you can defer across every 401(k), 403(b), and SIMPLE plan you participate in.¹ So if your W-2 employer’s plan takes $18,000 in deferrals during the year, your Solo 401(k) can only accept $6,500 more in employee deferrals.

Catch-up contributions add to the ceiling if you’re age 50 or older. The standard catch-up is $8,000, bringing your combined deferral limit to $32,500. Starting in 2026, participants who turn 60, 61, 62, or 63 during the year get a higher “super” catch-up of $11,250, pushing the total to $35,750.⁴ Catch-up dollars are shared across your plans the same way the base deferral is.

There’s a new wrinkle in 2026. If your FICA wages from your W-2 employer exceeded $150,000 in the prior year, any catch-up contributions must be designated as Roth rather than pre-tax. This mandatory Roth catch-up rule comes from Section 603 of the SECURE 2.0 Act and applies to catch-up dollars in both your employer plan and your Solo 401(k).

Tracking the combined total is on you. Your workplace payroll system has no view into what you contribute to your Solo 401(k), and your Solo 401(k) provider has no view into your workplace deferrals. Miss the cap and the consequences are ugly.

Employer Contributions Are Calculated Separately

On the employer side, your Solo 401(k) operates independently of anything your W-2 employer contributes. This is where the dual-plan strategy actually earns its keep.

How much you can put in as the “employer” depends on your business structure:

Sole proprietors and single-member LLCs can contribute up to 20% of net self-employment income, calculated after subtracting half of self-employment tax. The IRS computation effectively reduces the 25% statutory rate to roughly 20% for unincorporated owners.²
S-corporation owners can contribute up to 25% of the W-2 salary the business pays them.²

These employer contributions do not count against your $24,500 deferral cap, and they have no connection to any match or profit-sharing your W-2 employer puts into your workplace plan. The two businesses are unrelated entities making their own independent employer contributions.

The practical result: someone who spends the full $24,500 deferral at their day job can still make a substantial employer contribution through the Solo 401(k), funded entirely by side-business earnings. A sole proprietor netting $80,000 from side work could add roughly $14,900 in employer contributions on top of whatever the day-job plan already provides.

The Per-Plan Section 415 Ceiling

Each unrelated employer’s plan carries its own annual additions limit under 26 U.S.C. § 415(c). For 2026, total annual additions to a single defined contribution plan cannot exceed the lesser of $72,000 or 100% of the participant’s compensation from that employer.⁴⁵ This cap includes both the deferrals and employer contributions going into that plan.

Because Section 415 applies per plan when employers are unrelated, you can theoretically approach $72,000 in your Solo 401(k) and also receive substantial contributions in your employer’s plan. Combined savings across the two can exceed $72,000 for the year, which is the whole point of running both plans.

When Common Ownership Collapses the Two Plans Into One

This per-plan treatment only works if the two businesses have no ownership overlap. Under 26 U.S.C. § 414(b) and (c), businesses under common control are treated as a single employer for Section 415 purposes.⁶ If you own the side business and also hold a controlling ownership stake in the company sponsoring your W-2 plan, the IRS may aggregate the two. In that case the $72,000 ceiling applies across both plans combined rather than each separately.

Most people with a day job at a large company where they hold no ownership interest don’t need to worry about this. If you’re a significant shareholder in the employer that sponsors your W-2 plan, talk to a tax professional before assuming the two plans stand alone.

What Happens if You Blow Past the Deferral Cap

Going over the $24,500 combined deferral limit triggers a harsh outcome: the excess is taxed in the year you contributed it, and taxed again when you eventually withdraw it. Double taxation is the real cost, not a flat penalty.⁷

You can head off the second layer of tax by requesting a corrective distribution of the excess (plus any earnings on it) by April 15 of the year following the over-contribution. That deadline is firm and does not extend even if you file a tax return extension.⁷ The corrected amount is still counted in your income for the deferral year, but you avoid being taxed on it a second time at distribution.

Since payroll at your day job and your Solo 401(k) custodian have no line of communication, tracking sits entirely with you. The simplest habit: decide at the start of the year how to split the $24,500 between plans, then adjust in the fourth quarter once you know how your side income actually landed.

Setup and Funding Deadlines

To make Solo 401(k) contributions for a given tax year, the plan document has to be adopted by December 31 of that year. Funding can come later, but the paperwork must be signed and the plan officially established by year-end.

The contribution deadline itself is more generous. Employer profit-sharing contributions can be made up to the business’s tax filing deadline, including extensions. For sole proprietors, that’s typically April 15, or October 15 with an extension. S-corporations and partnerships face a March 15 deadline, extended to September 15 with a filing extension. Employee elective deferrals follow the same general deadlines, though the mechanics differ slightly since they’re theoretically withheld from compensation.

That timing matters if you start a side business late in the year. Launch in November, adopt the Solo 401(k) plan document before December 31, and you can fund employer contributions based on that year’s business income anytime before your filing deadline. That gives you well over a year to get the money in.

1
Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
2
Internal Revenue Service. One Participant 401k Plans
3
Office of the Law Revision Counsel. 26 USC 402 – Taxability of Beneficiary of Employees Trust
4
Internal Revenue Service. IRS Notice 2025-67 – 2026 Amounts Relating to Retirement Plans and IRAs
5
Office of the Law Revision Counsel. 26 USC 415 – Limitations on Benefits and Contribution Under Qualified Plans
6
Office of the Law Revision Counsel. 26 USC 414 – Definitions and Special Rules
7
Internal Revenue Service. Consequences to a Participant Who Makes Excess Deferrals to a 401(k) Plan