Key takeaways
- Self-employed investors choose a chassis, and the choice turns on contribution capacity, loans, and one tax acronym.
- A Solo 401(k) has a statutory exception that can neutralise UDFI on debt-financed real property; an IRA does not.
- The two can sometimes be combined, which is often better than treating it as an either-or.
Both a solo 401(k) and a self-directed IRA can hold alternative assets. For an owner-only business, choosing between them is less about what you can buy — the menus largely overlap — and more about four structural differences.
Contribution capacity
The solo 401(k) usually wins on sheer intake: it stacks employee deferrals on top of employer profit-sharing contributions, letting a strong-income year move far more into tax-advantaged status than IRA limits allow. Investors funding a large alternative purchase primarily from new contributions often land here for that reason alone.
Debt-financed real estate and UDFI
Here is the acronym: IRAs that buy real estate with non-recourse financing generally face UDFI — tax on the debt-financed slice of income. Qualified plans, including solo 401(k)s, enjoy a statutory exception for most acquisition debt on real property. For leveraged real estate specifically, that single difference often decides the question. The mechanics are in our UBIT/UDFI guide.
Loans, Roth options, and paperwork
A solo 401(k) can permit participant loans and can include a designated Roth account without income limits on those deferrals — flexibility IRAs don’t offer. The trade: a 401(k) is a plan you sponsor. It comes with plan documents to maintain, a filing requirement once assets pass the reporting threshold, and eligibility rules that change the moment you hire employees. A self-directed IRA is simpler to run and remains available regardless of employment structure.
Choosing — and combining
Favor the solo 401(k) when contribution capacity, leveraged real estate, or Roth deferrals drive the plan and you’ll remain owner-only. Favor the self-directed IRA for simplicity, rollover consolidation, or when employees are on the horizon. Many investors sensibly run both — the vehicles aren’t rivals so much as tools with different torque. Whichever chassis you choose, the prohibited-transaction rules ride along identically.
Educational only. This page is general information, not individualized investment, legal, or tax advice. Rules depend on your account type, transaction, tax year, and circumstances — consult a qualified professional.