Solo 401(k) vs self-directed IRA: which one for real estate?

What is a solo 401(k)?

A solo 401(k) is not a special product. The IRS calls it a one-participant 401(k) plan — "a traditional 401(k) plan covering a business owner with no employees, or that person and his or her spouse," with the same rules and requirements as any other 401(k) plan (IRS — one-participant 401(k) plans). Solo-k, Uni-k and individual 401(k) all mean the same thing.

Two features follow. Because you are both the employer and the only employee, you contribute in both capacities — an elective deferral out of your compensation and an employer contribution out of the business. And because there are no other common-law employees, there is no nondiscrimination testing, which is what makes a plan of this kind cheap enough for one person to bother with.

A "self-directed" solo 401(k) is the same plan with a document that does not restrict investments to a brokerage menu. The self-direction is a matter of plan drafting and where the trust account sits, not a separate section of the tax code.

Who qualifies — and what disqualifies you

The eligibility test has two halves, and both must hold.

• You need self-employment income. Net earnings from self-employment, or W-2 wages from your own S corporation. Sole proprietor, single-member LLC, partner, freelancer, consultant, real-estate agent — all fine. A side business alongside a day job counts, though the deferral limit is shared across every plan you participate in personally.
• You cannot have full-time common-law employees other than yourself and your spouse. Business partners and their spouses are fine. Independent contractors who are genuinely contractors are fine. The moment an eligible employee exists, the plan must cover them and the testing exemption disappears — the IRS is explicit that "the no-testing advantage vanishes if the employer hires employees." Note that long-term part-time rules can pull in employees you thought were excluded.

Rental income from properties you own personally is not self-employment income, and neither are interest, dividends or capital gains. Many people who want a solo 401(k) for real estate find that the real-estate income itself does not qualify them; some other activity has to.

A self-directed IRA has no eligibility test beyond having an IRA — any existing traditional, Roth, SEP or SIMPLE balance, or a rollover from an old employer plan, can fund one. That asymmetry is why this comparison is not one-sided: the plan is better on the merits and unavailable to many of the people asking.

Solo 401(k) vs self-directed IRA, side by side

Sources: IRS one-participant 401(k) plans; IR-2025-111 and Notice 2025-67; IRC §514(c)(9); Form 5500-EZ instructions. The full SDIRA rule set — custodians, disqualified persons, valuation — lives in the self-directed IRA guide.

The UDFI exemption is the headline difference

Retirement accounts are tax-exempt, but the exemption is not unconditional. Under IRC §§511–514, income an exempt account earns from debt-financed property is unrelated business taxable income in proportion to the debt. Buy a rental with a mortgage inside an IRA and a slice of the rent — and later a slice of the gain — is taxable to the IRA itself.

Section 514(c)(9) carves out an exception, and it is written for qualified organizations. That term covers a qualified trust described in §401 — which is what a 401(k) plan is — along with certain schools and pension trusts. It does not cover individual retirement accounts. So the same mortgage produces UDFI in an IRA and, subject to conditions, none in a solo 401(k) (26 U.S.C. §514(c)(9)).

The exception is conditional, not automatic. The main tests in §514(c)(9)(B): the purchase price must be fixed at acquisition; the amount or timing of debt service cannot depend on the property's revenue or profits, which rules out participating loans; the property generally cannot be leased back to the seller or bought from, or financed by, a disqualified person; and holding property through a partnership triggers additional tests, including the fractions rule. Sale-leaseback structures and seller financing are exactly where this exemption is lost.

Two further limits are worth stating plainly. The exemption addresses debt-financed income only — a plan that runs an active trade or business, such as habitual flipping that makes it a dealer, can still owe ordinary UBIT. And the loan must be non-recourse in both account types, because personally guaranteeing debt for your own plan or IRA is an extension of credit by a disqualified person and a prohibited transaction under §4975. The prohibited-transaction rules do not soften in a 401(k); the SDIRA pillar sets out the disqualified-person list that applies to both.

If you already hold leveraged property in an IRA and are weighing a Roth conversion of it, the timing mechanics are a separate subject — see Roth-converting leveraged real estate.

Participant loans: the plan can lend to you

An IRA can never lend money to its owner. Not a small amount, not briefly, not at a market rate — it is a prohibited transaction, and the consequence is that the account can be treated as distributed in full on 1 January of that year.

A 401(k) can, if the plan document provides for it. The limit is the lesser of $50,000 or 50% of the vested balance, generally repaid in level amortised payments at least quarterly over five years — longer for a principal residence — at a reasonable rate of interest (IRS — plan loan FAQs). For an investor whose plan holds property that is a real liquidity valve: an unexpected roof, a capital call, a gap between tenants. Miss the schedule and the balance becomes a deemed distribution, taxable and potentially subject to the 10% additional tax. It is credit, not free money.

No custodian — but checkbook control is not a licence

An IRA must be held by a bank or an IRS-approved non-bank custodian; §408 requires it. Investors who want to sign for the IRA's own transactions typically insert an LLC that the IRA owns and they manage — the "checkbook IRA" — which works but adds an entity, state filings and a live question about who is really directing the assets.

A 401(k) is already a trust. In a one-participant plan the owner is normally the trustee, so the plan can hold an ordinary bank or brokerage account in the trust's name and you sign on it. What you still need: an EIN for the trust, a compliant plan document, separate books, and independent year-end valuations for any asset without a market price.

What disappears is a review layer, not the rules — there is no custodian to refuse a transaction that looks like self-dealing, and §4975 is enforced after the fact, on audit, with the whole account at stake.

2026 contribution limits

This is the second reason the plan tends to win, and the gap is not close. The figures below are from IR-2025-111 and Notice 2025-67.

Three mechanics behind the table. The deferral limit is per person, not per plan — if you also participate in an employer's 401(k), the $24,500 is shared. For a sole proprietor or partner, the 25% employer contribution applies to "earned income": net self-employment earnings after deducting half your self-employment tax and the contribution itself, which lands near 20% of net earnings — the worksheets in Publication 560 do it properly. And a spouse who genuinely works in the business has their own deferral and annual-additions limit, which is how two-person households reach six-figure funding.

Roth deferrals inside the plan carry no income phase-out, which is the quiet advantage over a Roth IRA for higher earners. If the money is already sitting in the pre-tax side of a plan, converting it in place is a different route with its own tax bill — see in-plan Roth conversions — and the calculator will price the year either way.

Can you have a solo 401(k) and a SEP IRA?

You can hold both, but for one business they generally share a single ceiling rather than stacking. Contributions to a SEP established on the IRS model form are employer contributions to a defined-contribution plan, so your combined annual additions across the plans are tested against the same $72,000 §415(c) limit for 2026. Two genuinely unrelated businesses can sometimes support separate limits, but controlled-group and affiliated-service-group rules decide that, and they are unforgiving — get it reviewed rather than assumed.

In practice the solo 401(k) reaches the ceiling on far less income, because the employee deferral comes first and is not a percentage of anything. A SEP that pays $24,500 requires roughly $122,000 of compensation; a solo 401(k) can deposit the same amount from the deferral alone. The SEP's remaining merits are simplicity and a later deadline, not capacity.

Setting one up, and the provider landscape

1. Confirm eligibility — self-employment income this year, no full-time common-law employees other than a spouse.
2. Get an EIN for the business, and a separate one for the plan trust if your provider's document calls for it.
3. Adopt a plan document — an IRS pre-approved document or a custom one. If you intend to hold property, check specifically that it permits non-standard investments, and that it enables the features you want: Roth deferrals, participant loans, rollovers in.
4. Open the trust account in the name of the plan, titled correctly, with you as trustee. Nothing personal ever touches it.
5. Mind the deadlines. The plan generally has to exist before you can defer, and the deferral election has its own timing rules; employer contributions can usually be made up to the business's filing deadline including extensions. SECURE 2.0 gave sole proprietors extra room for first-year deferrals. Confirm the dates for your entity type.
6. Track the $250,000 line. Once plan assets reach that at year end you file Form 5500-EZ, generally by the last day of the seventh month after the plan year ends — and a final return is required for the year you terminate, whatever the balance.

On providers, this site makes no recommendations. The landscape splits into three groups. Brokerage and payroll-platform plans are free or near-free but almost always restrict investments to the platform's own securities — unsuitable for property. Document providers sell a pre-approved plan document with self-direction permitted, for a set-up fee plus an annual fee, and leave the banking to you. Third-party administrators cost more and do the compliance work: restatements, 5500-EZ preparation, loan amortisation. What to compare: whether the document truly allows non-standard assets, whether Roth and loans are enabled, who signs the 5500-EZ, and whether the fee is flat or scales with assets. Anyone promising the prohibited-transaction rules can be worked around is telling you something about themselves.

When the self-directed IRA still wins

• No self-employment income. The decisive case. An employee with a $600,000 rollover balance cannot open a solo 401(k) at all; the SDIRA is the only route to alternative assets.
• You have employees. One eligible full-time employee ends the one-participant plan, and a real 401(k) with testing and coverage obligations is a different cost base.
• Unleveraged assets. If you are buying all-cash, the §514(c)(9) advantage is worth nothing, and the IRA's lack of a plan document, trustee duty and 5500-EZ is a genuine saving.
• Non-real-estate alternatives. Private credit, tax liens, syndications and other debt-financed holdings that are not real property do not get the §514(c)(9) exemption in either wrapper.
• You want a Roth end state on existing IRA money, or you simply want the oversight — see the self-directed Roth IRA page for the destination account. Some investors are better served by a custodian who declines odd instructions.

Running both

These are not mutually exclusive, and the sequencing usually looks like this: leveraged real property goes in the solo 401(k) for the UDFI exemption and the borrowing capacity; unleveraged alternatives and older rollover money stay in the self-directed IRA, or move into the plan if the document accepts rollovers in. Note the direction of travel — you can generally roll a traditional IRA into a solo 401(k), but not a Roth IRA, which cannot be rolled into an employer plan at all.

Consolidating pre-tax IRA money into the plan has a side effect worth knowing: it removes that balance from the pro-rata calculation that governs backdoor Roth contributions. That is a planning consequence, not a reason on its own.