As a plan sponsor, you really need to let your third party administrator (TPA) what you have in terms of retirement plans and interrelated businesses.
Correct compliance testing is dependent on the TPA getting all the information you need. I’ll never forget when at a TPA I served as head ERISA attorney, we working on a defined benefit plan for a business owned by a famous politician. We did such a great job that they wanted us to handle their 401(k) plan. Of course, we asked: what 401(k) plan? Not letting a TPA know all about the retirement plans and businesses you own will make compliance testing meaningless because they’ll be wrong. The problem with incorrect compliance testing is that it may negatively impact the tax qualification of the plan, but it’s also something that may not be corrected for a number of years.
So it’s extremely important that you tell your TPA everything in connection with your plan that is relevant.