Solo 401(k) Deadline: When You Actually Have to Open the Plan
Pre-2019 you needed the plan open by Dec 31. SECURE Act killed that. Now: tax filing deadline including extensions.
One of the most-Googled Solo 401(k) questions: when's the deadline to open one? The answer changed in 2020 and a lot of articles online still cite the old rule. Here's the current state.
The old rule (pre-2020)
The plan had to exist by December 31 of the tax year you wanted contributions credited to. Miss the deadline by a day and the entire year was gone.
The SECURE Act 2019 change
The SECURE Act extended the plan setup deadline to your tax filing deadline including extensions. For 2026 contributions that means:
- Sole proprietor: Plan must exist by April 15, 2027 (or October 15, 2027 with an extension). Employer profit sharing has until the extended deadline; employee deferrals have until the unextended deadline.
- S-Corp: Plan must exist by March 15, 2027 (or September 15, 2027 with extension). Same employee/employer deadline split.
What "must exist" actually means
You need to have signed a plan adoption agreement and (depending on the provider) funded the plan with at least one dollar. You don't need to have made contributions yet. Most discount brokers (Fidelity, Schwab, E*TRADE) walk you through this in 30 minutes online.
Why this matters in practice
Did your tax preparer call in March 2027 with a $40k tax bill on your 2026 net SE earnings of $200k? You can still open a Solo 401(k) right now and shovel up to $46k of employer profit sharing into it before the extended deadline, dropping your tax bill by roughly $11,000 at a 24% marginal rate. The deadline extension turned the Solo 401(k) into a year-end tax tool you can use up to nine months after year end.
Common mistakes
1) Confusing the IRA deadline (April 15, no extensions) with the Solo 401(k) deadline (extensions count). 2) Funding the employer share before the employee deferral, both go to the same plan but get tracked separately on Form 5500-EZ. 3) Forgetting that S-Corp employee deferrals must be processed through W-2 wage reporting in the year they're earned; you can't retroactively defer wages.
Reading the fine print
The SECURE Act technically only allows post-year-end setup for the employer share. Employee deferrals require either an existing plan or, for sole props, the unextended due date. So the simplest play: open the plan now, even if you don't fund it, just to lock in flexibility.