Thanks MaryK4, but I am not sure that is always correct (in all situations)? In this specific case the Sole Proprietor already made the maximum allowable Salary Deferral Contributions to the Roth 401 K before making other After Tax Profit Sharing Contributions to the Traditional 401 K (and in 2022 Profit Sharing Contributions could only be made to the Traditional 401 K -- although SECURE 2.0 now allows Profit Sharing Contributions to Roth 401 K for 2023 and beyond - at least for those 401 K plans that incorporate these new rules). So (as I understand it) those After Tax Profit Sharing Contributions can't be deducted as Business Expenses (because they are for the benefit of the Business Owner) and they are of no value to the Owner as a Personal Deductions (because his tax liability is already zero). So I was just wondering if there is some way for him to count them as Cost Basis in the Traditional 401 K (and benefit from that in the future if/when he does In Plan Roth Conversions, as he expects to do)? Not sure that I quite understand the comment that "you cannot make nondeductible contributions and later convert to a Roth 401K". I think you could if (for example) you had made Salary Deferral Contributions to a Traditional 401 K (perhaps in excess of the Annual Maximum for Roth Contributions), and your 401 K Plan allowed In Plan Roth Conversions (but that is not really the question here). The question here is regarding Sole Proprietor's After Tax Profit Sharing Contributions to the Traditional 401 K, which can later be transferred into the Roth 401 K via an In Plan Roth Conversion (which the plan allows - although it is a taxable event). If there was no Business Deduction for the After Tax Profit Sharing Contribution (since it was for the Sole Proprietor) and no Personal Deduction for it either (since the Sole Proprietor already had a zero tax liability for 2022) can it be used to reduce the taxable amount of the future In Plan Roth Conversion? A fairly specific question I realize, and the short answer may just be "no" -- because (as you say) Self Employed 401Ks and IRA are not necessarily analogous.
