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I am trying to understand Solo 401k contribution deadlines for Single-Member LLC:

  1. Pre-tax contributions to traditional solo 401k. This includes both employer and employee contributions.

  2. After-tax contributions to Roth solo 401k. This also includes both employer and employee contributions.

  3. After-tax contributions to after-tax solo 401k and doing ‘mega backdoor Roth’. If I understand correctly After-tax contribution is employee contribution.

Also I was reading that contributions can be made before the business tax deadline, except for the Employee contribution part, which should be made before the end of the year:

“While employee and employer contributions may be extended until the company tax return deadline, you will typically need to file a W-2 for your wages (e.g. an S-Corporation) by January 31st, 2023. The W-2 will include your wage income and any deduction for employee retirement plan contributions will be reduced on the W-2 in box 12. As a result, you should make your employee contributions (up to $20,500 for 2022) by January 31st, 2023 or you should at least determine the amount you plan to contribute so that you can file an accurate W-2 by January 31st, 2023. If you don’t have all or a portion of the funds you plan to contribute available by the time your W-2 is due, you can set the amount you plan to contribute to the 401(k) as an employee contribution, and will then need to make a said contribution by the tax return deadline (including extensions).”

I am a bit confused about the W2 part. As a Single-Member LLC, I do not have a W2 form (I give my customers a W9 form when making contract work).

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    Where's the quote from? IIRC employer contribution (aka matching contribution) is always pre-tax traditional. But in your case you don't have employer contribution, you need to follow the "self-employed" instructions.
    – littleadv
    Oct 14, 2023 at 23:45

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As a self-employed individual reporting business income on Schedule C, you can disregard the instructions for W-2 employees, which would apply for Solo 401k participants who are paid a salary (i.e. S-corporations and C-corporations).

For a single member LLC taxed as a disregarded entity, the Solo 401k contribution deadline for "employee" elective deferrals (both pre-tax and Roth), employer nonelective profit-sharing, and voluntary after-tax contributions is your filing deadline plus extensions (if you claim an extension), as long as the plan was established by December 31 of the contribution year (e.g. December 31, 2023 for the 2023 tax year).

If the plan was established after December 31st of that year but before the tax return filing deadline, you are able to make employer profit-sharing and voluntary after-tax contributions only.

Note that the passage of SECURE 2.0 has greenlit Roth employer profit-sharing contributions starting in 2024. However, we're still waiting on guidance from the IRS on whether this will apply to employer contributions made during the calendar year 2024 (and could apply to the 2023 tax year), or if it will only apply to the 2024 tax year and later.

This all assumes you're eligible to have and participate in a Solo 401k. No 401k-eligible employees, etc.

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