Solo 401k Deadlines
Edited

Plan Setup

In most cases, you can set up a Solo 401k plan for a prior tax year as long as you have not yet filed your tax return. This is a relatively recent change, that was introduced by the SECURE Act.

The plan start date, which is a required field to set up a Solo 401k plan, will govern what years of compensation are eligible to be contributed. You cannot contribute dollars to a Solo 401k plan before the plan start date.

Contribution Deadlines

The deadline to contribute (deposit) new dollars (not rollovers) into your Solo 401k is generally the earlier of your tax filing due date, with extensions, and the date you file your return (you generally cannot contribute after you file your taxes).

Election Deadlines

Solo 401k contributions broadly fall into either employee contributions (employee pre-tax or Roth elective salary deferrals) or employer contributions (employer pre-tax profit-sharing contributions). You can read more about the differences here.

Employee contributions generally must be "elected," or decided, before 12/31 of each calendar year. For example, the deadline to elect a 2023 contribution was December 31, 2023. The election represents a ceiling, but not a floor - there is no penalty for electing an amount, but not fully funding it. You can make the election in the Carry application, or offline in your own records. The election does not need to be filed anywhere, it should just be kept on hand in case you are ever audited.

The one exception to the election deadline is for sole proprietorships in the first year of the plan. Recent legislation (SECURE 2.0) allowed for retroactive first-year elective deferrals, for sole proprietorships in their first plan year.

If you did not make an election for a prior year and are not eligible for the exception, you may be able to still contribute employer pre-tax contributions and employee voluntary after-tax (non-Roth) contributions, as these are not dependent on making an election.

In all cases, you should confirm that the amounts you are contributing are under your annual contribution limits, and that you accurately report your contributions on both your business/self-employment and individual tax returns.