The individual 401K plan is so popular because with small business owners because it is designed explicitly for small, owner only business.
Solo 401K plans have been designed for the self-employed business person and their spouse who have no full-time employees. Contributions to the plan are tax deductible like other self-directed IRA accounts, and the funds are considered to be assets that are intended to grow without being taxed directly as income or capital gains until you withdraw the funds from the account.
The Self Directed IRA Rules rules are extremely broad and the penalties extremely harsh (immediate disqualification of entire IRA plus penalty). Thus, an individual using a Self Directed IRA to make investments should be aware and cautious of the disqualified person and prohibited transaction rules.
Transfer retirement funds such as a Traditional IRA, SEP-IRA, SIMPLE IRA, 401 K, 403b, and most other pre-tax retirement plans tax-free from your current custodian to any financial institution or credit union who can serve as your Solo 401K Plan custodian for no fee.