Sole Prop - retro 401k set up

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Page 441 of General Explanation of Tax Legislation Enacted in the 117th Congress (December 2023)

17. Retroactive first year elective deferrals for sole proprietors (sec. 317 of the Act and sec. 401(b) of the Code)
Present Law
Present law provides a remedial amendment period during which, under certain circumstances, a retirement plan may be amended retroactively in order to comply with the tax qualification requirements.2030 Plan amendments to reflect changes in the law generally must be made by the time prescribed by law for filing the income tax return of the employer for the employer’s taxable year in which the change in law occurs (including extensions). The Secretary may extend the time by which plan amendments need to be made.  
Section 201 of the SECURE Act 2031 provides that if an employer  adopts a qualified retirement plan after the close of a taxable year but before the time prescribed by law for filing the return of tax of the employer for the taxable year (including extensions thereof), the employer may elect to treat the plan as having been adopted as of the last day of the taxable year. That provision permits employers to establish and fund a qualified plan by the due date for filing the employer’s return for the preceding plan year. However, that provision does not override rules requiring certain plan provisions to be in effect during a plan year, such as the provision for elective deferrals under a qualified cash or deferral arrangement (generally referred to as a ‘‘section 401(k) plan’’).
Under present law, if a section 401(k) plan is established by a sole proprietor after the end of the individual’s taxable year, then the plan can be funded with employer contributions as of the due date for the business’s return (including extensions), However, any election to make an elective deferral must be made by the end of the individual’s taxable year (i.e., generally by December 31 of the prior year). In contrast, an individual who contributes to an IRA is deemed to have made a contribution to the IRA for a taxable year if it is contributed after the taxable year has ended but is made ‘‘on account of’’ that year and before the due date for filing the IRA owner’s tax return for that year without extensions (generally, April 15).
Explanation of Provision
Under the provision, in the case of an individual who owns the entire interest in an unincorporated trade or business, and who is the only employee of such trade or business, any elective deferral 2034 under a section 401(k) plan to which the election under section 201 of the SECURE Act applies which is made by such individual is treated as having been made before the end of the plan’s first plan year if the election to make the elective deferral is made before the time for filing the return of such individual (determined without regard to any extensions) for the taxable year ending after or with the end of the plan’s first plan year. This extension of time would only apply to the first plan year the section 401(k) plan is established.
Effective Date
The provision is effective for plan years beginning after the date of enactment (December 29, 2022).