As a general rule, UBIT applies to both IRAs and solo 401k plans; however, UDFI applies to IRAs but not to solo 401k plans.
What is Unrelated Business Income Tax?
For example, if a self-directed solo 401k invests via an equity investment in a a shoe store or a computer store the income generated from the business would be subject to UBIT. Reason being, selling shoes an computer equipment or services is no the general purpose of an solo 401k plan. In order for not be subject to UBIT, the IRA or solo 401k investment must be passive in nature. This tax was created to keep tax-exempt entities on a level playing field with non-tax-exempt entities such as solo 401k plans and IRAs.
What is Unrelated Debt-Financed Income Tax?
Similar UBIT there is a another tax called Unrelated Debt-Financed Income (UDFI) tax that ONLY applies to IRA investment income derived from debt-financed property, proportionate to the debt on the property. HOWEVER, UDFI does not apply to solo 401k plans so this is why individuals prefer solo 401k plans over IRAs for investing in real estate whereby a non-recourse loan (debt financing) is used.