IRA Legal Structure: Trust, Custodian, and Investment Rules

The IRA legal structure is set by Section 408 of the Internal Revenue Code, which defines an Individual Retirement Arrangement as a trust (or a custodial account treated as one) created in the United States for the exclusive benefit of a single person or that person’s beneficiaries. It is a formal legal entity, not a brokerage or bank account with a tax label attached. If the arrangement fails any of the structural requirements built into the statute, it can lose its tax-exempt status entirely and expose the full balance to income tax.

The Trust Requirement in Section 408

Section 408(a) requires that every IRA be established through a written governing instrument that satisfies specific conditions.¹ That document is the legal backbone of the arrangement. It names the trustee, spells out how contributions and distributions will be handled, and incorporates the federal rules that keep the account tax-advantaged.

Several structural conditions have to appear in that instrument. The trustee must be a bank (defined broadly under the statute) or another entity that has proven to the IRS it can administer the trust. Contributions, aside from rollovers from other qualified retirement plans, have to be in cash. The owner’s interest must be nonforfeitable at all times. And the assets of the account cannot be commingled with other property except within a common trust fund or common investment fund.¹

Custodial Accounts Treated as Trusts

Most IRAs today are not technically trusts. They are custodial accounts, which Section 408(h) allows to be treated as trusts for tax purposes when the assets are held by a bank or another person who satisfies the IRS that the account will be administered consistently with Section 408. If those conditions are met, the custodian is treated as if they were a trustee.¹

The practical difference is administrative. A custodian holds and manages the assets on your behalf, executes trades, and files reports with the IRS, but does not hold legal title in the traditional trust sense. For the tax treatment, it does not matter. The custodial model is what allows online brokerages and large financial institutions to offer IRAs at scale without establishing a separate trust for each account holder.

Who Can Serve as Trustee or Custodian

Section 408(n) identifies three categories of institutions that automatically qualify:

Banks as defined under Section 581, which broadly covers corporations doing a substantial part of their business receiving deposits and making loans.
Credit unions insured under the Federal Credit Union Act.
Corporations subject to examination by a state banking commissioner or equivalent authority.¹

Non-Bank Trustee Approval

Entities outside those three categories can still serve as IRA trustees or custodians, but only after applying to the IRS and going through a rigorous review. Treasury Regulation 1.408-2(e) sets out the requirements. Applicants must demonstrate fiduciary ability, which includes showing they can continue operating regardless of ownership changes, maintain an established business location in the United States, and have sufficient experience administering retirement funds.²

The IRS also examines the applicant’s financial solvency, looking at net worth, liquidity, and ability to pay debts as they come due. Applicants have to show competence in accounting for large numbers of individual interests, collecting income, and safely keeping securities. The application requires a user fee based on the current revenue procedure, and processing can take considerable time.³ That gatekeeping is why the vast majority of IRAs end up at banks, credit unions, and brokerage firms that already meet the institutional qualifications.

Cash Contributions, Vesting, and Segregation

Three rules inside Section 408 define how money and assets have to sit inside the account. First, regular contributions must be made in cash. You cannot contribute appreciated stock, real property, or any other non-cash asset. The only exception is a rollover from another qualified retirement plan, which can move assets in kind.¹

Second, under Section 408(a)(4), your interest in the balance must be fully vested at all times. There is no vesting schedule of the kind that can apply to an employer match in a 401(k). Every dollar in the account belongs to you from the moment it arrives.¹

Third, Section 408(a)(5) requires that IRA assets not be mixed with other property. The only permitted exception is pooling assets through a common trust fund or common investment fund, the kind of collective vehicle a bank trustee might use internally. Outside of that, the IRA must remain a standalone pool of assets.¹ That segregation is what preserves the account’s tax-advantaged status and keeps retirement money clearly distinguishable from taxable holdings.

For 2026, the annual contribution cap is $7,500 for individuals under 50 and $8,600 for those 50 and older, applied across all your traditional and Roth IRAs combined rather than per account. Contributions also cannot exceed your taxable compensation for the year, and exceeding the cap triggers a 6% excise tax on the excess for each year it remains in the account.⁴

Investments the Structure Forbids

Life Insurance

Section 408(a)(3) prohibits investing any IRA funds in life insurance contracts. If IRA money is used to purchase life insurance, that portion is treated as a taxable distribution to the account owner.¹

Collectibles

Section 408(m) prohibits IRAs from acquiring collectibles. The statutory definition is broad: artwork, rugs, antiques, gems, stamps, coins, alcoholic beverages, and any other tangible personal property the Secretary of the Treasury specifies.⁵ When an IRA buys a collectible, the purchase price is treated as a distribution in the year the item is acquired, meaning immediate income tax at ordinary rates plus a possible 10% early withdrawal penalty for owners under 59½.⁶

The Precious Metals Exception

Congress carved out a narrow exception for certain coins and bullion. An IRA can hold U.S. gold, silver, and platinum coins minted by the Treasury, plus gold, silver, platinum, or palladium bullion that meets the minimum fineness a regulated futures exchange requires for delivery. Gold must be at least .995 fine; silver at least .999 fine. The bullion has to be in the physical possession of the IRA’s trustee, not stored at home or in a personal safe deposit box.⁷

Prohibited Transactions and Disqualified Persons

Section 4975 restricts who can do business with an IRA. It identifies a category of “disqualified persons” that includes the IRA owner, the owner’s spouse, ancestors, lineal descendants, their spouses, any fiduciary of the IRA, and anyone providing services to it.⁸ Common examples of prohibited transactions include borrowing from the IRA, selling property to it, using it as security for a personal loan, and buying property with IRA funds for personal use. The IRS looks at both direct and indirect transactions, so routing a deal through a family member or a controlled entity does not avoid the rule.⁹

The consequence is severe. Under Section 408(e)(2), if the owner or beneficiary engages in a prohibited transaction at any point during the year, the account stops being an IRA as of January 1 of that year. The entire balance is then treated as a distribution at its fair market value on that date. Income tax applies to the full amount, and the 10% early withdrawal penalty applies on top of that if the owner is under 59½.¹ Self-directed IRA owners run into this most often. A single transaction with a disqualified person can wipe out years of tax-deferred growth in one stroke.

Where the Roth IRA Fits

A Roth IRA, created under Section 408A, uses the same trust-or-custodial-account structure as a traditional IRA. The rules on qualified trustees, cash-only contributions, prohibited investments, asset segregation, and prohibited transactions all carry over. The differences sit on the tax side rather than in the legal chassis.¹⁰ Contributions are never deductible, qualified distributions come out tax-free after the account has been open five tax years and a triggering event has occurred, and there are no required minimum distributions during the original owner’s lifetime. Roth IRAs also carry income-based eligibility limits that traditional IRAs do not, while sharing the same $7,500 base contribution cap for 2026.

1
Office of the Law Revision Counsel. 26 USC 408 – Individual Retirement Accounts
2
eCFR. 26 CFR 1.408-2 – Individual Retirement Accounts
3
Internal Revenue Service. Application Procedures for Nonbank Trustees and Custodians
4
Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
5
Legal Information Institute. 26 USC 408(m)(2) – Collectible Defined
6
Internal Revenue Service. Retirement Topics – Exceptions to Tax on Early Distributions
7
Legal Information Institute. 26 USC 408(m)(3) – Exception for Certain Coins and Bullion
8
Internal Revenue Service. Retirement Topics – Prohibited Transactions
9
Office of the Law Revision Counsel. 26 USC 4975 – Tax on Prohibited Transactions
10
Office of the Law Revision Counsel. 26 USC 408A – Roth IRAs