If the benefit plan of the single employer, was adopted by affiliate companies (via adoption agreement or board action) the benefit plan would still be treated as a single employer plan requiring only one filing for Form 5500.
After the document is generated, minor alterations would be needed. Primarily the definition of Employer would need to be amended to include the employer and “any affiliate of the controlled group who formally adopts the plan”. Secondly a separate Appendix would have to provided which would list all of the affiliate companies, by name, that have adopted the plan and any exceptions such as if the adopting employer can pick and choose benefits. Finally a section, describing how the affiliate companies will be charged back to pay for their plan participation, should be added.
Aside from these changes, the plan itself is still treated as a single employer plan. Usually the employer acting as plan sponsor designates the eligibility rules and similar rules for all participating employers. The employer can provide the affiliate with several options in regards to the plan benefits such as opting out of certain coverage’s, or an “all or nothing” approach.