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Can Catch-Up Contributions Go Into the Voluntary After-Tax Solo 401k Bucket?
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1 Reply
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I’m 55 years old and I want to maximize the Mega Backdoor Roth Solo 401k strategy. I know I’m eligible for a catch-up contribution on top of the regular limits. Can I put my $8,000 catch-up contribution into the voluntary after-tax holding account as part of the Mega Backdoor Roth? Or does it have to go somewhere else?
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This important and often misunderstood IRS rule directly: voluntary after-tax Solo 401k contributions can never be made as catch-up contributions. This is a firm IRS regulation — not a plan-specific restriction.
The voluntary after-tax contribution only applies to the overall §415(c) annual additions limit — which is $72,000 for tax year 2026. Catch-up contributions exist outside and above this limit — they are a separate layer that the IRS does not permit to be directed into the voluntary after-tax holding account.
So for a participant aged 55 in 2026, the correct contribution structure is:
- The $8,000 catch-up contribution must be made to the pre-tax Solo 401k or the Roth Solo 401k employee holding account — never to the voluntary after-tax holding account
- The voluntary after-tax contribution for the Mega Backdoor Roth Solo 401k strategy is calculated on the remaining headroom within the $72,000 overall limit — after employee deferrals and employer profit-sharing are accounted for
Key Rule: The maximum that can be contributed to the voluntary after-tax Solo 401k holding account in any year is the §415(c) overall limit ($72,000 for 2026) minus all other annual additions (employee deferrals + employer profit-sharing). Catch-up contributions are completely separate and always go to the pre-tax or Roth employee holding account — never the voluntary after-tax bucket.
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