SDIRA vs. Solo 401(k): Which Account Should Hold Your Note Money?

You already decided you want to be the bank instead of the landlord. Good. Now comes the boring question that quietly decides how much of the money stays yours.

Which account holds the note?

Most people pick this backwards. They open whatever account a buddy opened, fund it, buy something, and then find out about the tax rules on the back end. That is an expensive way to learn.

So here is the head to head. Self-directed IRA versus Solo 401(k), scored on the five things that actually matter when you are lending retirement money into mortgage notes: how much you can put in, how fast you can move, what the IRS taxes on the way through, how badly you get punished when you slip, and how much paperwork you just signed up for.

One housekeeping note before we start. This is education, not tax or legal advice. Run your version of this by a CPA who has actually seen a self-directed account before, not one who is going to Google it while you are on the phone.

Two Accounts, One Job

A self-directed IRA is still just an IRA. Same rules, same limits, same tax treatment as the one at your brokerage. The difference is the custodian. A self-directed custodian will actually hold a mortgage note, a deed of trust, or an LLC interest instead of telling you they only do mutual funds. Legally it is a trust under section 408, the custodian holds title, and you direct where the money goes. If you want the full mechanics, start with how a self-directed IRA actually works.

A Solo 401(k) is a real qualified retirement plan under section 401, built for an owner-only business. You sponsor it, you are usually the trustee, and the plan itself owns the assets. No custodian standing in the middle.

Both can buy a performing note. Both can buy a non-performing note. Both grow tax-deferred, or tax free if you are running the Roth side. On that level they do the same job.

The differences are all in the plumbing. And the first one is a gate, not a preference: you cannot open a Solo 401(k) without self-employment income. A 1099, an LLC, an S-corp, a side business that actually earns. Not a hobby you gave a name. And it has to stay owner-only: no full-time common-law employees besides you and your spouse. Hire one real employee and your Solo 401(k) eligibility is done.

Contribution Room: The Solo 401(k) Wins By a Mile

Here are the 2026 numbers, straight from the IRS cost-of-living adjustments in Notice 2025-67.

Self-directed IRA: $7,500 for the year. Add $1,100 if you are 50 or older. So $8,600 is the ceiling.

Solo 401(k): you wear two hats, and both get to contribute.

• As the employee, you defer up to $24,500.
• Add $8,000 in catch-up if you are 50 or older. If you happen to be 60 through 63, that catch-up is $11,250 instead.
• As the employer, your business kicks in a profit-sharing contribution on top, calculated off your compensation.
• Total annual additions cap out at $72,000, with the catch-up sitting on top of that.

$8,600 versus $72,000. That is not a close fight.

By all means, do not let that decide it by itself. Contribution room only matters if you have income to contribute. If your actual plan is to move a $200,000 rollover out of an old employer plan and go buy notes with it, the annual limits are noise. Rollover capacity is effectively unlimited in both accounts, and that is where most first-time note money comes from anyway.

One trap worth knowing. A traditional IRA can generally be rolled into a Solo 401(k). A Roth IRA cannot be rolled into the Roth side of one. That door only opens one way, so think before you consolidate.

Checkbook Control: How Fast Can You Actually Fund a Deal?

This is the one nobody warns you about until a tape drops.

With an SDIRA, the custodian holds the asset, so the custodian signs. Every purchase means a direction-of-investment form, a document review, and a wire request that lands in a queue. Best case, a couple of days. Real case, sometimes a week. There are per-asset annual fees riding along too, which is a real cost when you are stacking a bunch of small notes instead of one big building. If you are comparing shops on that math, we broke it down in the 2026 custodian comparison.

Note deals do not always wait a week. Bids get due Friday.

The usual workaround is an IRA-owned LLC, what people call a checkbook IRA. The IRA owns the LLC, you manage it, and you sign at the LLC level. It works. It also adds legal setup cost, an annual state filing, and more surface area for mistakes.

The Solo 401(k) gets checkbook control by design. You are the trustee. You open a bank or brokerage account in the name of the plan, and you sign and wire yourself. No custodian in the loop, no form, no queue.

Now the honest part, because this cuts both ways. Checkbook control means there is nobody standing between you and a prohibited transaction. That slow custodian who keeps asking annoying questions is also a speed bump against a dumb move. When you remove the referee, you have to be the referee.

UBIT and UDFI: The Part Most People Get Wrong

Start with the good news, because it is bigger than the scary news.

Interest income is generally excluded from unrelated business taxable income. That is section 512(b)(1). Buy a note with all cash inside either account, collect the interest, and you are not looking at a UBIT bill. Same answer in an SDIRA. Same answer in a Solo 401(k). For the plain vanilla version of being the bank, this whole section is a non-issue. That is what people miss when forum posts scare them off.

Two things break that.

1. Debt financing (UDFI). If the account borrows money to acquire an income-producing asset, the slice of income attributable to the borrowed portion becomes taxable under section 514. Buy with the account's own cash, no issue. Buy with a non-recourse loan, and part of that income gets pulled into the tax net.

2. Running a business instead of holding an investment. Take a property back through foreclosure, rehab it, sell it, and do that over and over inside the account, and at some point that stops looking like passive investing and starts looking like a trade or business. That income can be taxable at trust rates, which reach the top bracket at a painfully low income level.

Here is where the two accounts genuinely split. Section 514(c)(9) carves out debt-financed real property from UDFI, but only for what the code calls a "qualified organization." That definition includes a qualified trust described in section 401. A Solo 401(k) is one of those. An IRA is a section 408 trust, so it is not on the list.

Practical translation: a Solo 401(k) using a non-recourse loan to buy a rental property can often sidestep UDFI. An SDIRA doing the identical deal generally cannot.

Two cautions before you go build a strategy on that, though.

• The carve-out is for real property. A mortgage note is a debt instrument, not real property. If you are borrowing to buy notes, do not assume the exception covers you. Ask first.
• Even when it applies, section 514(c)(9)(B) lists conditions that blow the exception anyway. Price not fixed at acquisition, certain seller financing, participating loan features. It is not a free pass, it is a narrow door.

So for a note buyer paying cash, this round is mostly a tie. For someone who also wants debt-financed real property sitting in the same account, the Solo 401(k) advantage is real and it is worth money.

Prohibited Transactions: Same Rules, Very Different Bill

Section 4975 applies to both accounts, and the list of disqualified persons is the same: you, your spouse, your parents and grandparents, your kids and grandkids and their spouses, and entities you control. You cannot lend the account's money to them. You cannot buy an asset from them. You cannot personally benefit from an account asset.

In note land that means no swinging a hammer yourself on a house the account took back. No paying the servicer's fee out of your personal checking to save a step. No writing your kid a note out of the account at a friendly rate. The rules are not complicated. They are just absolute.

Where the accounts split is the punishment, and it is not close.

IRA: under section 408(e)(2), a prohibited transaction stops the account from being an IRA as of the first day of that tax year. The whole balance gets treated as distributed to you. Tax on all of it, plus the early withdrawal penalty if you are under 59½. One bad move, entire account, gone.

Solo 401(k): section 4975 hits the disqualified person with an excise tax. 15% of the amount involved, and 100% if you do not correct it in time. That is an ugly bill and a correction you have to actually make. It is not the whole plan detonating.

To be honest, that asymmetry is the most persuasive argument in the whole comparison. Run a live note portfolio with servicers, foreclosures, and workouts, and you will have more chances to get something wrong. The account with the softer failure mode is worth something.

Setup Friction and the Paperwork You Will Actually Do

SDIRA setup: pick a custodian, open the account, fund it by transfer, rollover, or contribution. Days to a couple of weeks, mostly waiting on the sending institution. You pay an annual custodial fee plus per-asset fees. If the account ever does owe UBIT, the custodian files the Form 990-T and pays it out of the account. You personally file nothing. That is the whole appeal.

Solo 401(k) setup: more moving parts.

• You need real self-employment income, and you need to keep needing it.
• You need a plan document, an EIN for the plan, and a trust account at a bank or brokerage.
• You have to stay owner-only. Watch part-timers too. Long-term part-time employee rules can pull people into eligibility you did not plan for.
• Once plan assets pass $250,000 at year end, you file a Form 5500-EZ every year. Plus a final one when you close the plan.
• Plan documents get restated periodically. Somebody has to actually do that, and that somebody is you or whoever you pay.

The Solo 401(k) gives you more room and more control. It also hands you more homework. Pick honestly, because an unmaintained plan is a worse outcome than a boring custodian.

Head to Head, Side by Side

Which One Fits Which Investor

Strip out the noise and it comes down to who you are, not which account is "better."

W-2 employee, no side income, sitting on an old 401(k) or IRA balance. Self-directed IRA. You cannot open the Solo 401(k) anyway, and the contribution limits are irrelevant to you because you are deploying a rollover, not making new contributions. Simplest path from where you stand to an IRA holding mortgage notes.

Self-employed with real net income, owner-only. Solo 401(k). More room, faster funding, softer failure mode on a prohibited transaction, and the option to borrow from the plan if life happens. The paperwork is the price of admission.

Planning to use non-recourse debt on real property inside the account. Solo 401(k), because of the 514(c)(9) exception. This is the single biggest tax divergence between the two, and it is worth real dollars over time.

Buying notes all cash and want the least possible admin. Self-directed IRA. The UBIT edge does not apply to you, so you are paying Solo 401(k) complexity for a benefit you will not use.

Spouse works in the business with you. Solo 401(k). Both of you can participate, which roughly doubles the annual room without hiring anybody.

Might need to touch the money before retirement. Solo 401(k). A participant loan up to 50% of your vested balance, capped at $50,000, is available. The same move inside an IRA is a prohibited transaction that vaporizes the account.

Nobody Says You Have to Pick Just One

This gets framed as a fight and it usually is not. Plenty of self-employed investors run a Solo 401(k) for new contributions and anything bought with debt, then keep a self-directed IRA on the side for all-cash notes where custodial simplicity is fine.

The Roth angle is worth a conversation with your CPA too. Roth deferrals inside a Solo 401(k) have no income phase-out. Roth IRA contributions phase out between $153,000 and $168,000 for single filers in 2026. And under SECURE 2.0, designated Roth accounts inside a plan no longer have required minimum distributions during your lifetime, which changes the legacy math. If that is new to you, read up on how RMD rules work now.

Here is the thing to keep straight through all of it. The account is a wrapper. It does not create the return. The note does that, and a note that does not perform is a bad note in any container you put it in.

What the wrapper decides is how much you keep, how fast you can move when a good one shows up, and how much a mistake costs you. Those three things are worth getting right before you wire anything.

So which one are you actually working with right now, the rollover or the side business income?

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Related Reading

• how real estate and notes work inside a self-directed IRA
• how a self-directed IRA works
• putting IRA money into mortgage notes

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AJ Dent is the founder of Take Notes Capital, a mortgage note investing firm specializing in non-performing notes. Book a free strategy call.

Educational content only. Nothing here is tax, legal, or investment advice, and nothing here is an offer to sell or a solicitation to buy any security. Contribution limits and tax figures cited are the IRS 2026 amounts from Notice 2025-67. Confirm current figures and your own situation with a qualified CPA or attorney.