Self-Employed Retirement: Solo 401(k) vs. SEP IRA

For independent contractors, freelancers, and small business owners, selecting the right retirement vehicle is a primary lever for tax optimization. While both the Solo 401(k) and the SEP IRA offer tax-deferred growth, their contribution structures and administrative requirements differ significantly.

1. 2024-2025 Contribution Limit Comparison

The most critical factor for high earners is the "Contribution Ceiling." The Solo 401(k) typically allows for higher contributions at lower income levels because it combines employee and employer components.

Feature	Solo 401(k) (2024)	Solo 401(k) (2025)	SEP IRA (2024)	SEP IRA (2025)
Employee Deferral	$23,000	$23,500	N/A	N/A
Catch-up (Age 50+)	$7,500	$7,500	N/A	N/A
Employer Max	25% of Compensation	25% of Compensation	25% of Compensation	25% of Compensation
Total Limit (under 50)	$69,000	$70,000	$69,000	$70,000

Concrete Example: The $100k Earner

If a freelancer earns $100,000 (net of SE tax):

SEP IRA: Can only contribute $20,000 (25% of compensation, after adjustments).
Solo 401(k): Can contribute $23,000 (employee) + $20,000 (employer) = $43,000.
Advantage: The Solo 401(k) allows the freelancer to shelter more than double the income compared to the SEP IRA at this income level.

2. SECURE Act 2.0 and the "Roth" Shift

The SECURE Act 2.0 (passed late 2022) introduced two major changes for the self-employed:

Roth SEP IRAs: Previously, SEP IRAs were strictly pre-tax. Now, owners can elect to treat contributions as Roth (after-tax), allowing for tax-free withdrawals in retirement.
Roth Employer Contributions: Employers can now make matching or non-elective contributions to a Solo 401(k) on a Roth basis, provided the employee is 100% vested.

3. Structural Differentiators

The Solo 401(k) (The "Power User" Plan)

Loans: You can borrow up to 50% of the balance (max $50,000) for any reason.
Roth Option: Built-in Roth component for employee deferrals.
Deadlines: Must be established by Dec 31, but contributions can be made until the tax filing deadline.

The SEP IRA (The "Simplicity" Plan)

Setup: Extremely easy to set up (often just a single form at a brokerage).
Flexibility: You can skip contributions in lean years without penalties.
Deadlines: Can be set up and funded as late as the tax filing deadline (including extensions).

4. Summary Decision Matrix

Choose Solo 401(k) If...	Choose SEP IRA If...
You want to maximize contributions at <$250k income.	You prioritize low administrative overhead.
You want a Roth component for your deferrals.	You are past the Dec 31 deadline for the current year.
You need the ability to take a plan loan.	You have employees (Solo 401k is only for owner/spouse).