A controlled group 401(k) problem almost never announces itself. The plan runs fine for years, then the owner buys a second company, spins out a management entity, or brings a partner’s practice under the same roof — and suddenly the employees of that other entity may legally belong in your coverage and nondiscrimination testing, whether or not anyone enrolled them.
Under ERISA and the Internal Revenue Code, related employers can be treated as a single employer for retirement plan purposes.
Why the rules exist at all
Qualified plans get favorable tax treatment on the condition that they benefit a broad cross-section of employees, not just owners and executives. The related-employer rules close that door by aggregating businesses under common ownership or common service arrangements and asking the coverage question across the whole group.
The practical consequence for sponsors: your plan’s testing population is defined by corporate structure, not by which entity signs the paychecks or which entity adopted the plan document.
Controlled groups: the ownership test
1. Parent-subsidiary
One entity owns a controlling interest in another, or a chain of entities connects through a common parent.
2. Brother-sister
The same small group of individuals owns controlling interests in two or more entities, and their ownership overlaps meaningfully across those entities. This is where most surprises live, because nothing on the org chart connects the companies — only the shareholder list does.
3. Combined groups
A mix of the two, where a brother-sister group also has a parent-subsidiary chain hanging off it.
Two details trip sponsors up more than the base definitions. First, ownership can be attributed: interests held by a spouse, child, parent, trust, estate or partnership may be counted toward an individual even though that individual holds nothing directly. Family-owned businesses are the classic case — two siblings who each run "their own" company may still be one employer after attribution.
Affiliated service groups: the services test
Affiliated service group (ASG) rules exist because ownership is not the only way to fragment a workforce.
An ASG generally involves a service organization together with another organization that either regularly performs services for it, or performs services for its clients, in a way that makes the businesses functionally interdependent.
ASG analysis is legal analysis, not arithmetic. Sponsors should not self-determine it from a spreadsheet.
What changes when another entity is in the group
Once entities are aggregated, several plan mechanics move at once:
- Coverage testing is run against all employees of the group, including those of an entity that never adopted the plan. Excluded employees are not invisible; they count as non-benefiting.
- Nondiscrimination testing (ADP/ACP and, where applicable, general testing) uses the aggregated population.
- Top-heavy determination can shift, sometimes dramatically, when a second entity’s owners join the highly compensated population.
- Service and eligibility crediting follows the employee across group entities — an employee who transfers from one member to another generally does not restart their eligibility or vesting clock. See our guide to 401(k) eligibility and entry dates.
- Annual limits that apply per employer are applied at the group level, which affects owners who draw compensation from more than one member.
- Form 5500 reporting and the participant count that drives the audit requirement reflect the plan’s actual population, not the sponsoring entity’s headcount alone.
How another entity actually joins your plan
Aggregation and participation differ. Being in a controlled group means the other entity’s employees must be counted. It does not automatically mean they are covered. Coverage requires a document step.
- The related entity is formally added as a participating employer under the plan document, usually through a participating employer agreement or adoption resolution signed by both entities.
- The adoption agreement’s eligibility, exclusion and entry provisions are reviewed to confirm they produce the intended result for the new population.
- Payroll for the new entity is connected to the recordkeeper feed, with its own division or location coding so deferrals and match can be traced.
- Prior service with the acquired or related entity is credited according to the document’s service-crediting language.
Skipping step one is the most common failure. Employees get enrolled and deferrals start flowing because payroll was configured, while the plan document never named their employer — which means contributions were accepted for people the plan does not cover.
Where sponsors get hurt
- An acquisition closed and nobody told the TPA. Transition relief for coverage after a corporate transaction is limited and conditional; treating it as an indefinite pass is how a coverage failure compounds across years.
- The census only covers one entity, so testing runs on an incomplete population and looks clean while it is wrong.
- Family attribution was never applied. The ownership questionnaire was answered from the cap table, not from the family tree.
- A management company was created for tax reasons and no one asked what it does to plan aggregation.
- Ownership changed mid-year — a buyout, a new partner, a gift of shares — and the group determination was never rerun.
- Two entities run two plans and each is tested on its own, when the group must be tested together or the plans aggregated.
A self-check any sponsor can run this quarter
- List every legal entity the owners hold an interest in, including inactive, holding and management entities.
- For each one, record the individual owners, their percentages, and their family relationships to owners of the other entities.
- Flag any entity that provides services to, or receives services from, another entity on the list.
- Compare that list to the participating employers named in your plan document and to the entities appearing in your census file.
- Reconcile the census headcount to the total W-2 count across all group entities — unexplained gaps are the signal.
- Ask your TPA in writing which entities were included in last year’s coverage and nondiscrimination testing.
- Document the conclusion, with the date and the person who verified it, in your annual plan review file.
A group determination is a point-in-time answer, and a plan sponsor’s fiduciary and settlor records should show when it was last confirmed — the same discipline that belongs in your year-end compliance calendar.
What you can delegate — and what you cannot
You can delegate the operational work: collecting the ownership data, building a complete multi-entity census, running and documenting testing, tracking participating employer agreements, and reconciling payroll feeds across entities. A full-scope 3(16) plan administrator should be doing all of it, and coordinating with your TPA so nothing falls between the two.
You cannot delegate the underlying facts. Only the owners know what they own and who they are related to. Sign the ownership questionnaire carelessly and every downstream test inherits the error — and the sponsor remains responsible for the accuracy of the data given to service providers. Professional and closely held businesses face this most often; see our pages for medical practices, professional services firms, and closely held businesses.
Who signs for the result
Whatever the structure turns out to be, someone has to stand behind the filing that reports it. Think about that before you sign the Form 5500 for a plan whose testing population you have never independently verified.
Admin316 accepts the ERISA 402(a) named fiduciary appointment — the role most providers decline — alongside full-scope 3(16) administration.
If your company owns, controls or manages more than one entity, the group question is worth answering deliberately rather than discovering it during an audit. Schedule a plan review with Admin316 and we will walk your structure against your plan document.
