Which Is Better for Q1 Tax Savings on Side-Business Income, Solo 401(k) or SEP IRA?
Side-business income in Q1? Solo 401(k) stacks employee deferral + employer contribution for maximum savings; SEP IRA offers simple employer-only funding with later deadlines. Model AGI/QBI impact and coordinate with your day-job plan.
Summary of What This Blog Covers
- Who qualifies for Solo 401(k) vs SEP IRA in real life + Q1 gotchas
- 2026 contribution math, timing rules, and coordination with day-job plans
- Impact on AGI, QBI, NIIT, backdoor Roths, and cash flow with practical scenarios
Who Truly Qualifies for Each Plan (Real-Life Gotchas)
Solo 401(k): no full-time employees (other than owner/spouse). Must have self-employment income. Can miss deferral if deadline passed.
SEP IRA: simpler, no employee deferral, can fund for prior year by tax deadline (extensions). Gotcha: no Roth, no loans, no catch-up if over 50 without deferral option.
2026 Contribution Math & Timing Rules
Solo 401(k): employee deferral $24,500 + employer 25% of comp → total up to $72,000 ($80,000 if 50+ catch-up). Deferral by Dec 31; employer by tax deadline.
SEP IRA: employer-only, up to 25% of comp or $72,000. Deadline: tax filing (incl. extensions). No deferral or Roth.
Coordinating with a Day-Job Plan
Day-job 401(k) limit is separate. Solo 401(k) deferral counts against your personal $24,500 limit. SEP IRA employer contribution is separate. Model combined limits to avoid over-contribution.
What Your Choice Does to AGI, QBI, NIIT & Backdoor Roth
Solo 401(k) deferral reduces AGI → helps QBI phase-out, NIIT threshold, backdoor Roth eligibility. SEP IRA reduces AGI but no deferral control. Solo 401(k) often better for high earners.
Solo 401(k) vs SEP IRA Decision Checklist (copy-paste)
☐ Projected side-business profit modeled
☐ Can I afford deferral by Dec 31?
☐ Want Roth option?
☐ Spouse participating?
☐ Need loan feature?
☐ Day-job 401(k) limit coordinated
☐ AGI/QBI/NIIT/backdoor impact checked
Book a Fractional CFO Strategy Session
Insogna models AGI/QBI impact, coordinates with any workplace 401(k), and implements before deadlines. We pick the right plan and lock in the savings. Whether you’re searching for a tax preparer, CPA in Austin, or tax advisor near you, book a consultation and make your side-income work smarter this year.
Frequently Asked Questions
1) Can I contribute to both a day-job 401(k) and Solo 401(k)?
Yes — employee deferral limit ($24,500 in 2026) is personal. Employer contributions are separate per plan.
2) SEP IRA deadline — can I fund for 2025 in 2026?
Yes — by your tax filing deadline (including extensions), often Oct 15, 2026.
3) Solo 401(k) — can I do Roth deferral?
Yes — Roth employee deferral available (tax-free growth). SEP IRA is traditional only.
4) Which saves more on taxes?
Solo 401(k) usually — deferral reduces AGI immediately + higher total contribution. Model your income.
5) Spouse — can they contribute?
Solo 401(k): yes, if bona fide employee with W-2 wages. SEP IRA: yes, treated as self-employed.
