A solo 401(k) (one-participant 401(k)) has two funding sides with different calendars: employee elective deferrals (traditional or Roth if the plan allows) and employer profit-sharing. Many freelancers assume a SEP-style extension window reopens both sides. In practice, elective deferrals usually must follow the plan’s year-end / election rules, often tied to December 31 for a calendar-year sole prop, while employer contributions in many years can still land by the business tax filing deadline including extensions. Dual-hat overview: Solo 401(k) basics.
Providers such as Fidelity, Schwab, Vanguard, and E*TRADE need the deposit typed as employee deferral vs employer and labeled for the correct tax year. Tax filing habits: Filing taxes for beginners. Investing basics for beginners.
Three clocks people mix up
| Clock | What it controls | Typical cue (verify the year’s IRS / plan rules) |
|---|---|---|
| Employee elective deferral election | Last day the owner can elect the deferral amount for that tax year | Generally by December 31 of that year for sole props (plan document controls) |
| Employee elective deferral deposit | Last day elected deferrals can be funded to the account | For many owner-only plans, timely elected deferrals may be deposited by the business return due date including extensions, do not conflate election day with deposit day (confirm Pub 560 / prototype) |
| Employer profit-sharing deadline | Last day the business can fund the employer side for that year | Often the business return due date with valid extension |
| Shared elective deferral cap | How much employee deferral room you have across all 401(k)/403(b)/similar plans | Day-job Fidelity/Vanguard 401(k) deferrals count against the same employee cap |
A September SEP deposit habit does not automatically create a September “catch-up elective deferral” into a solo 401(k). Confirm Publication 560, your prototype adoption agreement, and the year’s IRS one-participant plan FAQs before you copy a blog date.
Compare nearby plan calendars: SIMPLE IRA basics (payroll remittance + employer match clocks) and SEP IRA contribution deadline basics (employer-only style funding). HSA cousin: HSA contribution deadline basics.
Practical funding steps
- Adopt / keep a written solo 401(k) prototype before you need to fund for the year.
- Decide employee deferral amount (and Roth vs traditional split) in time for the plan’s election deadline, do not wait for an October extension mindset.
- Track any W-2 401(k) deferrals so the shared employee cap is not double-counted (Solo 401(k) basics).
- Calculate employer profit-sharing from net self-employment compensation worksheets; fund by the applicable employer deadline and label the tax year at the custodian.
- Keep the adoption agreement, deferral election, and ACH/wire confirmations with the tax file.
- When plan assets cross Form 5500-EZ thresholds, calendar that filing separately from contribution deadlines.
Worked example: deferral locked Dec 31, employer funded in September
Cameron runs a design LLC with no employees and a Schwab solo 401(k). For tax year 2025, Cameron elects $15,000 employee deferrals (some Roth) in writing by December 31, 2025. Day-job 401(k) deferrals already used part of the shared employee cap, so Cameron stops at $15,000 rather than assuming full solo room. Because the election was timely, Cameron funds the $15,000 deferral by the plan’s allowed deposit deadline (same-year is simplest; some owner-only designs allow funding through an extended return due date, confirm before waiting).
On September 10, 2026, after an extension and final Schedule C math, Cameron adds an $8,500 employer profit-sharing contribution labeled tax year 2025. That September deposit is the employer clock. Cameron cannot add another 2025 employee deferral in September just because the SEP neighbor still had room, the elective deferral window for 2025 already closed under Cameron’s plan.
If the September wire had been mistyped as a 2026 employee deferral, both years’ records and the Form 5500-EZ / return positions could be wrong.
Common pitfalls
- Skipping a year-end election, then treating solo 401(k) employee deferrals like SEP employer money you can invent in September.
- Ignoring the shared elective deferral cap with a day-job plan.
- Funding before the plan is formally adopted for that year.
- Labeling employer profit-sharing as “employee deferral” (or the reverse) in the custodian UI.
- Skipping Roth vs traditional election records when the plan allows both.
Checklist
- Separate employee deferral deadlines from employer profit-sharing deadlines on one calendar.
- Confirm the plan’s elective deferral cutoff (often year-end for solo calendar-year plans).
- Coordinate the shared employee deferral cap with any W-2 401(k).
- Use extension season for employer funding, and for employee deposits only when a timely year-end election already exists, not to create a late election.
- Label every custodian deposit with the correct tax year and contribution type.
- Re-read the prototype and IRS pubs for the year you are funding.
Educational only. Not tax, legal, or investment advice. Contribution limits, deferral election rules, and employer deadlines change by tax year, verify with IRS publications, the plan document, or a qualified tax professional.