Any business with employees that have either: Reached the age 21 and/or Worked 1,000 or more hours in the year. Solo 401(k) plans can be amended and upgraded to an employee-based plan in the event you add employees. Tip: If you, other owner, or a spouse in this plan owns another business(es), you may have to cover all these employees in a plan and would need to open a different plan type.