Title I of the Employee Retirement Income Security Act (ERISA) protects employees in workplace plans: fiduciary duties, disclosure to participants, reporting to the Department of Labor, and its enforcement. It applies only to plans that cover employees.
A Labor Department regulation, 29 CFR 2510.3-3, says who does not count as an employee for this purpose. An individual and their spouse are not employees of a business, incorporated or not, that is wholly owned by the individual or by the individual and spouse. Partners and their spouses are not employees of the partnership. A plan that covers only these people is outside Title I.
It is still a qualified plan under the Internal Revenue Code. The written plan document, the 415(b) limit, minimum funding and the annual Form 5500-EZ all apply. Separately, the PBGC's pension insurance does not cover a plan maintained only for substantial owners, such as the sole owner of the business, so there are no PBGC premiums. The trade-off is the combined deduction limit when you also have a 401(k).
The status can change. Once a common-law employee becomes a participant, Title I applies. A corporation owned by two unrelated people is not described in the regulation, and whether its plan is outside Title I is not settled. See who can have a solo cash balance plan and hiring your first employee.