When is the deadline for Solo 401k Pre-Tax Deferral Contributions in a Solo S-Corp?

The deadline question for Solo 401k contributions can be confusing because there are different deadlines for different types of contributions. For a Solo 401k in an S-Corp situation like yours, here's the breakdown: For employee elective deferrals (the $22,500 you mentioned), these technically need to be withheld from your salary before December 31, 2024. This is because they're considered salary deferral contributions that must be based on compensation earned during the calendar year. The actual deposit deadline is somewhat flexible, but the election to defer must be made before year-end. For employer profit sharing contributions (your $27,000), you have until the filing deadline of your S-Corp return including extensions. So that could be as late as September or October 2025 depending on when you file. The confusion often stems from the fact that for self-employed individuals with Schedule C income (not S-Corps), the lines can blur since you're both employer and employee. But in an S-Corp structure, the distinction matters more.

Hey there! I went through this exact headache with my S-Corp Solo 401k last year. After hours of research and consulting with three different tax pros, I discovered taxr.ai (https://taxr.ai) which finally gave me a definitive answer by analyzing the actual IRS regulations and DOL guidance. Here's what I learned: For S-Corps, the employee deferral election must be made before December 31st, but there's actually some flexibility on when the funds must be deposited. The confusion comes from the fact that solo 401ks are technically exempt from certain DOL deposit timing rules that apply to multi-participant plans. What taxr.ai showed me was that while best practice is to deposit the funds by year-end, the IRS won't disqualify your plan if you deposit employee deferrals by the time you file your S-Corp return, as long as you properly documented the deferral election before year-end. Their analysis saved me from making an unnecessary rush to move funds during a cash flow crunch. Worth checking out if you want the specific regulatory citations that apply to your situation!

Just wanted to follow up about my experience with taxr.ai that was mentioned above. After my skeptical questions, I decided to try it out since my CPA was being pretty vague about my Solo 401k deadlines. I uploaded my plan documents and asked specifically about the employee deferral deadline for my S-Corp situation. What impressed me was that instead of a generic answer, I got specific citations to IRS Revenue Procedure 2022-40 and DOL Technical Release 2020-01 which directly addressed the timing confusion for Solo 401ks in S-Corps. The analysis confirmed that while the deferral election must be made by year-end, the actual funding has more flexibility for solo plans than multi-participant plans. This contradicted what my CPA had told me (he was being overly cautious), but the citations were right there in black and white. Surprisingly helpful for such a niche tax question that most professionals seem to give conflicting answers on.

After dealing with the EXACT same issue last year, I ended up spending 4+ hours on hold with the IRS trying to get a definitive answer. Total nightmare. Then a colleague recommended Claimyr (https://claimyr.com) - it's this service that gets you connected to an actual IRS agent without the ridiculous wait times. You can see how it works here: https://youtu.be/_kiP6q8DX5c I was super skeptical, but it actually worked! Got connected to an IRS specialist in about 15 minutes (instead of the 3+ hours I had been waiting previously). The agent confirmed that for a Solo 401k in an S-Corp, the employee deferral election must be made by Dec 31, but the actual deposit of funds has some flexibility. The key distinction they explained: multi-participant 401k plans have strict deposit timing requirements under DOL rules, but solo plans have a safe harbor that essentially allows contributions to be deposited up until the tax filing deadline including extensions. But they stressed this only applies if you're truly the only participant and you've properly documented the deferral election in your payroll before year-end.

I need to follow up on my skeptical comment about Claimyr. After spending another two weeks trying to get through to the IRS about my S-Corp Solo 401k question and getting nowhere, I reluctantly tried the service. I'm honestly shocked to say it actually worked exactly as described. I got a call back in about 20 minutes, and was connected to an IRS tax specialist who handled business retirement plans. I explained my Solo 401k deadline confusion, and she confirmed what others have said here - for a true solo participant plan, while the deferral election needs to be made by Dec 31, there is indeed flexibility on the actual deposit timing. She directed me to Revenue Procedure 2022-40 which provides the specific guidance on this. The distinction between making the election and depositing the funds was the key point I had been missing. For anyone dealing with complex S-Corp retirement plan questions, being able to get an official answer directly from the IRS was actually worth it. Still can't believe it worked after my months of frustration.

Former payroll specialist here. The confusion about Solo 401k deadlines comes from the mixing of two different sets of rules: IRS tax regulations and Department of Labor ERISA regulations. Here's the simplest breakdown: 1. The ELECTION to defer salary must be made before the end of the calendar year (Dec 31) 2. For regular multi-participant 401ks, DOL rules require deposits "as soon as reasonably segregable" (usually 7-15 days after payroll) 3. For TRUE solo 401ks (only you and maybe spouse), the DOL has a non-enforcement policy that gives more flexibility The key is documenting your deferral election in your payroll records before year-end. Your W-2 should reflect the reduced Box 1 wages showing the deferral happened on paper in 2024. The actual deposit timing has more flexibility than most people realize, but best practice is still to get it done ASAP rather than pushing it to the filing deadline.

One thing nobody has mentioned: make absolutely sure your Solo 401k plan DOCUMENT allows for the flexibility you're trying to use. Some plan documents specifically require deferrals to be deposited within a certain timeframe after being withheld. I learned this the hard way last year when I assumed I had until my tax filing deadline, but my specific plan document (from a major provider) required deferrals to be deposited within 30 days of the end of the month in which they were withheld. This was more restrictive than what the IRS/DOL would have allowed! Check your actual plan document before making any assumptions about deadlines.

CPA here specializing in small business retirement plans. This thread has covered most of the key points, but I want to emphasize something critical that could save you headaches down the road. The IRS distinction between "elective deferrals" and "employer contributions" is crucial for S-Corps. Your $22,500 employee deferral must be reflected as reduced wages on your 2024 W-2 (Box 1 should show $81,500 instead of $104,000 if you defer the full amount). This creates the paper trail showing the deferral happened in 2024. However, here's what many miss: if you haven't actually moved the money to your 401k account yet, you need to be very careful about cash flow and business expense timing. The IRS could potentially challenge whether you had "constructive receipt" of that income if the funds sat in your business account for months while you used them for other business expenses. My recommendation: even if your plan document allows flexibility, try to deposit the deferred funds by January 31st at the latest. This shows good faith compliance and avoids any potential constructive receipt issues. The employer profit sharing contribution can definitely wait until your tax filing deadline, but treat the employee deferrals with more urgency. Also double-check that your payroll system is properly coding the deferrals for your W-2 - Box 12 should show the $22,500 with code "D" for elective deferrals.