Form 5500-EZ is the annual return a one-participant plan files with the IRS. A one-participant plan covers only the owner, or the owner and spouse, of a wholly owned business, or only partners and their spouses. Such a plan cannot file Form 5500 or 5500-SF instead.
A return is due for any year in which the plan's assets, added to the assets of all the employer's other one-participant plans, are more than $250,000 at the end of the plan year. A Solo 401(k) and a cash balance plan count together. A return is always due for the final plan year, when all assets are paid out, whatever the balance. A cash balance plan funded at six figures a year usually crosses the threshold in its second year.
It is due on the last day of the seventh month after the plan year ends, which is July 31 for a calendar-year plan. Form 5558 extends that by 2½ months. The penalty for filing late is $250 a day, up to $150,000 per plan year. Returns go on paper or electronically through EFAST2, and one-participant filings are not published online.
The plan administrator or employer signs it, which in a one-person plan is you. The actuary's Schedule SB is not attached but must be kept. See the Form 5500-EZ guide.