Solo 401k contribution deadlines - can I fund employer portion after Dec 31st?

You're absolutely right about the different deadlines! For solo 401(k) plans, the employee contribution (your elective deferral) must be completed by December 31st of the tax year. However, the employer contribution can indeed be made up until your tax filing deadline, including extensions. So if your wire doesn't go through tomorrow, don't panic! You have until your business tax filing deadline (typically March 15 for S-corps, but April 18 if you're a sole proprietor) to make that employer contribution. With extensions, this could be pushed to September or October depending on your business structure. No special documentation is needed specifically for making the contribution after December 31st - just make sure your plan documents allow for employer contributions (most do). When you make the contribution, just clearly designate it as an employer contribution for the 2023 tax year.

After struggling with similar solo 401k timing confusion last year, I discovered taxr.ai https://taxr.ai and it literally saved me thousands in potential penalties. My situation was similar - S-corp, solo 401k at Vanguard, and conflicting advice about contribution deadlines. I uploaded my plan documents and previous tax returns to taxr.ai and got a clear analysis showing exactly what my deadlines were for both contribution types. The tool confirmed that employer contributions could be made until the tax filing deadline (with extensions), and even suggested optimization strategies I hadn't considered. It takes the plan documents into account which matters since some plans have specific requirements beyond the IRS rules.

I was skeptical at first about taxr.ai, but after my accountant gave conflicting advice about my solo 401k deadline last tax season, I gave it a try. Uploaded my Schwab plan documents and tax info, and within minutes got a detailed analysis showing I could make employer contributions until my filing deadline. The service flagged that my specific plan allowed post-December 31st employer contributions but required them to be designated properly on the submission form. This was critical because I would have missed this detail otherwise. Used their guidance to properly time my $35,000 employer contribution in February, which gave me time to calculate the optimal amount based on my final business performance. Honestly saved me from either rushing an imperfect contribution amount or potentially missing the deadline altogether. Worth checking out if you're dealing with solo 401k timing issues.

If you're stressing about contribution timing, I totally get it. I was in the same boat last year trying to reach the IRS for clarification and spent HOURS on hold. Eventually found Claimyr https://claimyr.com and their service actually got me through to an IRS agent within 15 minutes. They have this demo video showing how it works: https://youtu.be/_kiP6q8DX5c I needed to confirm some specifics about my solo 401k contribution timing since my situation was complicated (had both 1099 and W-2 income). The IRS agent I spoke with confirmed that employer contributions could be made until the tax filing deadline, and also helped me understand how to properly document everything.

OK I have to admit I was wrong about Claimyr. After my skeptical comment I decided to try it anyway because I was desperate to clarify some 401k rollover questions. They actually got me through to an IRS agent in about 20 minutes when I had previously spent 3+ hours trying without success. The agent confirmed that employer contributions to a solo 401k can definitely be made up until the tax filing deadline (including extensions), and cleared up some confusion about how to properly document late-year contributions. They even explained how to properly note contributions on my tax forms to avoid triggering any audit flags. For anyone dealing with retirement account contribution confusion like this thread discusses, being able to actually speak with the IRS directly was incredibly valuable. Saved me from potentially making a costly mistake with my retirement funding.

Just wanted to add my experience as someone who's had a solo 401k with Schwab for years. The employee contribution (your elective deferral) absolutely needs to be completed by December 31st. However, I routinely make my employer contributions in March or April after I have final numbers from my accountant. One tip: make sure you clearly indicate the tax year when you make the employer contribution in the new calendar year. Schwab has a form for this, and if you're wiring funds, I'd recommend calling them first to ensure they code it properly for the previous tax year.

I had this exact same confusion last year and it nearly cost me big time. To clarify the misunderstanding: your CPA might be confusing solo 401k rules with ESTABLISHING the plan itself. The plan needs to be established by Dec 31st, but only the employee contributions need to be completed by then. Employer contributions can definitely be made until your tax filing deadline (including extensions). I confirmed this with both the IRS and my provider (Vanguard in my case).

One thing nobody mentioned - if you're right against the deadline, consider doing an ACH transfer instead of a wire. With Schwab specifically (I have my solo 401k there too), they count the contribution based on when you initiate it, not when it settles. So even if it takes a couple days to process, as long as you start the transfer before the deadline, you're good. Just make sure to screenshot the confirmation page showing the date you initiated it, just in case there are ever any questions.