Form 5558 is filed to apply for a one-time extension of time to file Form 5500, which covers retirement plans. A separate Form 5558 must be filed for each plan for which an extension is requested. For example, if an employer maintains a defined benefit plan and a 401(k) plan, a separate Form 5558 must be filed for each plan. Form 5558 must be filed by July 31st for the prior calendar year. It allows an extension of 2 1/2 months or up to October 15th to file the final 5500.