SEP-IRA
A Simplified Employee Pension (SEP) is the easiest small-business retirement plan to set up. Employer contributions only (no employee deferrals), up to 25% of compensation (20% effective rate for self-employed after the SE tax adjustment), capped at the annual defined-contribution limit. Funded as late as the extended due date of the return.
SIMPLE IRA
Available to employers with 100 or fewer employees who do not maintain another retirement plan. Employees defer up to the SIMPLE limit; employer must either match dollar-for-dollar up to 3% of compensation or contribute a nonelective 2% of compensation to all eligible employees. Lower contribution limits than 401(k) but much simpler administration.
Solo 401(k)
A 401(k) plan covering only an owner (and optionally a spouse) with no other employees. Combines employee elective deferrals (up to the 401(k) limit) with employer profit-sharing contributions (up to 25% of compensation), often producing the largest possible total contribution for self-employed individuals. Roth and after-tax mega-backdoor sub-accounts are available with the right plan documents.
Qualified defined-benefit and cash balance plans
For high earners over 50 with strong cash flow, a defined-benefit or cash balance plan can permit annual deductible contributions far exceeding defined-contribution limits — sometimes $200,000+ per year. Higher administrative cost (actuary required) but unmatched tax shelter.
Deadlines
SEP-IRAs can be established and funded as late as the return due date (with extensions). SIMPLE IRAs must be established by October 1 of the year of contributions. Solo 401(k)s must be established by year-end (under SECURE Act, plan can be adopted by the return due date for prior-year employer contributions, but elective deferrals require year-end adoption).
Companion forms and schedules
Most IRS publications are written to support one or more specific forms. The publication's first chapter typically lists the forms it covers, and the form instructions cross-reference the publication. Used together, the form instructions and the publication answer most line-by-line questions. When they conflict — which is rare but happens, usually after a mid-year legislative change — the form instructions generally win, because they are revised more frequently and reflect the most current IRS interpretation.