If you run a small business with a combined cash balance and 401(k) plan, you’ve probably heard about the Mega Backdoor Roth. It’s a popular strategy among high-income earners at large companies, and it’s easy to see the appeal: a way to get significantly more money into a Roth account than the standard 401(k) deferral limit allows.
So it’s a reasonable question to ask your TPA or advisor: “Can I add this to my plan?”
For most of our clients with a cash balance and 401(k) combination, the honest answer is no — or at least, not without taking on more risk and complexity than the strategy is worth. Here’s why.
What a Mega Backdoor Roth Actually Is
A Mega Backdoor Roth isn’t a special plan type. It’s a strategy that layers on top of an existing 401(k) plan, and it depends on the plan document specifically permitting two features:
- Voluntary after-tax employee contributions — a separate contribution source from your regular pre-tax or Roth deferrals, and
- A mechanism to move that after-tax money into a Roth account — either an in-plan Roth conversion feature or an in-service withdrawal to a Roth IRA.
The basic idea is to fill up the remaining room under the IRC Section 415(c) annual additions limit — which caps total contributions from all sources (employee deferrals, employer contributions, and after-tax contributions) — with after-tax dollars, then convert those dollars to Roth. For 2026, that overall limit is $72,000 (this figure is indexed for inflation).
If you’re a business owner with no employees — a true Solo 401(k) — this can work reasonably well. There’s no one else to worry about, and the plan can be designed around your own contribution capacity.
Add employees to the picture, and the analysis changes substantially.
Why Employees Change Everything
The feature that makes a Mega Backdoor Roth possible — voluntary after-tax employee contributions — is not protected by your plan’s Safe Harbor status. Safe Harbor contributions exempt your plan from ADP testing on regular deferrals, but after-tax contributions are a separate source, and they’re generally subject to their own nondiscrimination test: the ACP test (Actual Contribution Percentage test).
In plain terms, the ACP test compares how much highly compensated employees (HCEs) — typically the owner and any other high earners — are benefiting from after-tax and matching sources, against how much your non-highly compensated employees (NHCEs) are benefiting from those same sources. If the owner puts in a large after-tax contribution and the staff doesn’t participate in a comparable way, the plan can fail the test.
This is where the mismatch really shows up in a small plan. With only a handful of participants, there isn’t much room for the numbers to average out. One or two NHCEs choosing not to make after-tax contributions — which is the norm, since most employees don’t have the cash flow or motivation to fund a voluntary after-tax bucket — can be enough to fail the test on its own.
A failed ACP test isn’t just an inconvenience. It typically means some or all of the owner’s after-tax contribution has to be refunded, along with associated earnings, and reported as taxable income. In other words, you’ve gone through the trouble of setting up the strategy just to reverse it.
Existing Contributions Don’t Solve the Problem
A common misconception is that a well-funded plan — one that already includes Safe Harbor and profit-sharing contributions — has enough of a cushion to absorb this. It generally doesn’t. Safe Harbor and profit-sharing contributions address different tests; they don’t create ACP testing room for a separate after-tax source. The after-tax contribution has to stand on its own for ACP purposes.
There’s also a second layer to consider when your 401(k) is paired with a cash balance plan: the combined plan deduction limit under IRC Section 404(a)(7). This section governs how much a business can deduct across a 401(k) and defined benefit plan (like a cash balance plan) together. Adding a large after-tax contribution to the mix doesn’t just raise a testing question — it can also affect how contribution and deduction room is allocated across the two plans. This interaction is plan-specific and depends on your actual contribution formulas, so it needs a case-by-case review rather than a general rule.
A Simplified Example
Say a business has an owner and three staff members. The owner wants to contribute $25,000 in voluntary after-tax dollars to chase Roth space, on top of the plan’s existing Safe Harbor match and profit-sharing contribution. None of the three staff members elect to make after-tax contributions of their own — which, again, is the typical outcome.
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When the ACP test is run, the owner’s after-tax contribution is compared against the (in this case, zero) after-tax contributions from the NHCE group. Without NHCE participation to support it, there’s a good chance the test fails, and some or all of that $25,000 has to come back out, likely alongside earnings that then get taxed.
This is a simplified illustration, not a projection for any real plan — actual results depend on compensation levels, existing plan contributions, and how many NHCEs are in the plan.
What This Means for Your Plan
For a small plan with any NHCEs, a Mega Backdoor Roth generally isn’t a good fit alongside a cash balance and 401(k) design. The compliance risk — and the possibility of contributions being refunded after the fact — usually outweighs the tax benefit the strategy is meant to provide.
If you want to understand the underlying Mega Backdoor Roth mechanics in more detail, particularly how it can work well for an owner-only Solo 401(k), we cover that in a separate article.
Talk to Your 401(k) TPA First
One important note: Emparion typically administers the cash balance side of these arrangements, not the 401(k) plan itself. Your 401(k) TPA is the right party to confirm exactly how your plan document currently handles voluntary after-tax contributions, whether a Roth conversion feature is even in place, and how ACP testing would apply to your specific census. We’d recommend looping them in before pursuing this strategy any further.
