For W-2 employees running a side consultancy, freelance practice, or small business on the side, the retirement account question becomes a fork in the road: SEP-IRA or solo 401(k)?
SEP-IRAs get marketed as simpler, but that simplicity costs you tens of thousands in tax-advantaged space at moderate-to-high self-employment income. Here’s where the crossover happens, with real numbers for the $80k+ net SE income earner.
The Fundamental Structure Difference
SEP-IRA
- Single contribution: up to 25% of net SE income (after half of SE tax)
- 2026 max: $70,000 (indexed)
- No employee-style deferral
- One-page Form 5305-SEP; file with Schwab, Fidelity, or Vanguard in 15 minutes
Solo 401(k)
- Employee deferral: up to $23,500 ($30,500 if 50+)
- Employer profit-share: up to 25% of SE income on top
- 2026 total max (pre-50): $70,000
- Plan document required; most brokers offer free prototypes
Both max out at $70k, so what’s the difference? When the same $70k max applies.
The Crossover Point: Why Solo 401(k) Wins at Moderate Income
To hit $70k in a SEP, you need roughly $280k of net SE income (25% of $280k = $70k).
To hit $70k in a solo 401(k), you need net SE income of about $190k: $23.5k employee deferral + 25% × $186k profit-share ≈ $70k.
Worked Example: $100k Net SE Income
SEP-IRA contribution:
- Net SE income (after 1/2 SE tax): ~$93k
- Max contribution: 25% × $93k ≈ $18.6k
Solo 401(k) contribution:
- Employee deferral: $23.5k
- Employer profit-share: 25% × $93k ≈ $18.6k wait, it’s actually 20% of net SE income before SE tax deduction for sole props; the formula is ~18.6% of gross SE income. Let’s say ~$17k.
- Total: ~$40.5k
Same side income, solo 401(k) contributes $22k more. At 37% federal marginal rate that’s $8k+ in current-year tax savings.
The Employee Deferral Caveat: W-2 Coordination
If you’re maxing a W-2 401(k) at your day job, you’ve already used the $23.5k employee deferral limit. The employee portion of the solo 401(k) is off the table because the $23.5k is an IRS-level annual limit across all employee deferrals, not per account.
In that case, solo 401(k) comparisons shift:
- Solo 401(k) employer profit-share only: same as SEP essentially
- SEP-IRA: same cap, simpler paperwork
So the answer for a W-2-maxed primary employee with side income: SEP or solo 401(k) profit-share produce similar results; pick based on other factors below.
But if you have unused W-2 deferral room (not maxing your day-job 401(k)), or if you don’t have a W-2 401(k), solo 401(k) captures that extra $23.5k.
The Backdoor Roth Killer: SEP-IRA’s Hidden Cost
SEP-IRA balances count toward the pro-rata rule for backdoor Roth conversions. If you have $50k sitting in a SEP and do a $7k nondeductible IRA contribution plus conversion, the IRS treats the conversion as a proportional mix of pre-tax SEP + after-tax contribution.
Result: the backdoor Roth becomes partly taxable. High earners routinely use the backdoor Roth strategy and this pro-rata pollution is often a dealbreaker.
Solo 401(k) balances do not count in the pro-rata calculation (they’re 401(k) assets, not IRA). This alone is why most high earners pick solo 401(k) over SEP-IRA.
Mega Backdoor Roth via Solo 401(k)
The real reason sophisticated side-earners run solo 401(k)s: Mega Backdoor Roth potential.
If your solo 401(k) plan document allows after-tax contributions and in-plan Roth conversions (or in-service distributions), you can contribute beyond the $70k main limit through after-tax dollars, then immediately convert to Roth. Practical max: another $15-30k Roth per year depending on plan design.
Custodian reality in 2026:
- Fidelity: prototype solo 401(k) does NOT allow after-tax or Mega Backdoor
- Schwab: same, prototype-only, no Mega Backdoor
- E*TRADE: prototype-only, no Mega Backdoor
- Vanguard: added Roth solo 401(k) but still no after-tax Mega Backdoor in prototype
- Third-party administrators (MySolo401k, Employee Fiduciary): custom plan docs with Mega Backdoor provisions, ~$500-1,500 setup + annual fees
If Mega Backdoor matters to you, budget for a TPA. Prototype plans from big brokers lock you out.
Spouse on Payroll: Doubling Capacity
If your spouse works in the business (even part-time administrative), you can put them on W-2 payroll and they get their own solo 401(k). Double the employee deferral space ($47k combined) and additional profit-share.
This requires bona fide employment: reasonable wages, documented hours, legitimate work. Don’t pay a spouse $50k for “family bookkeeping” if they actually do 2 hours a month.
The S-Corp Layer
At $150k+ net SE income, electing S-Corp status and paying yourself reasonable W-2 plus K-1 distributions can add SE tax savings on top of retirement account optimization. But the solo 401(k) calculation changes: employer contribution is now 25% of W-2 wages, not net SE income. This means smaller W-2 = smaller profit-share room.
Optimizing the W-2 vs K-1 split for maximum 401(k) contribution while minimizing SE tax is a multi-variable problem. It’s worth a CPA consult above $200k SE income.
When SEP Still Wins
SEP-IRA is the right choice when:
- SE income is sporadic and you don’t want annual Form 5500 filing (solo 401(k) requires this once plan assets exceed $250k)
- You already have significant pre-tax IRA balances and don’t care about backdoor Roth
- Simplicity is worth more than the extra tax-advantaged space
Everyone else: solo 401(k), ideally with a TPA if Mega Backdoor Roth is part of your plan.
For the broader picture on side-income structure, the side hustle tax/retirement stack pulls this together with S-Corp and other layers.
The side-income retirement decision is not about “$70k cap is the same either way.” It’s about how much income you need to hit that cap, what you sacrifice on backdoor Roth, and whether Mega Backdoor is on the table. At $80k net and up, the answer is almost always solo 401(k) with the right plan document.