What Is a Controlled Group 401(k)? Types, Tests, and Risks

A controlled group 401(k) is what results when the IRS treats two or more businesses under common ownership as a single employer for retirement plan purposes, forcing every employee across all the linked entities to be counted together when the plan is tested for compliance. The rules live in IRC Sections 414(b) and 414(c), and they exist to keep owners from stacking generous benefits inside one company while ignoring rank-and-file workers in another they also own.¹ Whether your setup falls inside the definition turns on ownership percentages, and the answer changes what your plan has to do to stay qualified.

The Three Kinds of Controlled Groups

Section 414(b) pulls in corporations that meet the controlled group definition in IRC Section 1563(a). Section 414(c) extends the same treatment to partnerships, sole proprietorships, and LLCs under common control. The analysis looks at equity, voting power, and profits interests. Options and warrants count as if already exercised.² Three structures can result.

Parent-Subsidiary

A parent-subsidiary group forms when one business owns at least 80 percent of another, measured by combined voting power or by total share value. Only one measure needs to be met.³ Once the first 80 percent link is in place, any further entity that is 80 percent owned by a group member joins the chain. A parent that owns 80 percent of Subsidiary A, which owns 80 percent of Subsidiary B, creates a three-member group even though the parent has no direct stake in B.

Brother-Sister

Brother-sister groups are the structure most small business owners run into. They exist when five or fewer people (individuals, estates, or trusts) own a controlling interest in two or more businesses, and two tests are both satisfied.

The controlling interest test requires that the same five-or-fewer owners together hold at least 80 percent of the voting power or total value of each business.⁴

The identical ownership test looks at each owner’s stakes across the businesses and counts only that person’s smallest percentage in any one of them. Add those minimums together across all the owners; if the sum is more than 50 percent, the test is met. So an owner holding 60 percent of Company A and 25 percent of Company B contributes 25 percent to this calculation, not 60.⁴

Both tests have to be passed. A dentist who owns a practice and a separate rental LLC with similar ownership stakes is the classic case.

Combined

A combined group arises when three or more businesses are each already part of a parent-subsidiary or brother-sister group, and at least one entity is both the common parent of a parent-subsidiary chain and a member of a brother-sister group. That dual role bridges the two structures into one aggregated employer.³

Family Attribution Can Put You In a Group You Didn’t Know You Were In

Ownership on paper is not always ownership as the IRS counts it. Under IRC Section 1563(e), shares owned by certain family members are attributed to you, and that attributed ownership can push you across the thresholds above.

For minor children under 21, attribution is automatic and runs both directions: parent to child and child to parent.³

For adult children, parents, grandparents, and grandchildren, attribution only kicks in once you already own more than 50 percent of the corporation (direct ownership plus anything attributed from your minor children). Once you clear that threshold, their shares are added to yours.

For spouses, each is generally treated as owning the other’s stock. The attribution can be avoided, but only if all four of these conditions are met: the spouse owns no direct stock in the corporation, is not a director or employee and doesn’t participate in management, no more than 50 percent of the corporation’s gross income comes from passive sources (rents, royalties, dividends), and the stock carries no restrictions in favor of the other spouse or their minor children.

For estates and trusts, stock is attributed to any beneficiary with an actuarial interest of at least 5 percent, in proportion to that interest. Stock in a grantor trust is treated as owned by the grantor.

A practical illustration: you directly own 40 percent of a partnership and your 20-year-old son directly owns 30 percent. You’re treated as owning 70 percent (your 40 plus his 30, because he’s a minor). Since that exceeds 50 percent, your 30-year-old adult son’s 20 percent now attributes to you as well, taking your constructive ownership to 90 percent.² That is more than enough to link this partnership to another business you own.

Affiliated Service Groups Are a Separate Trap

Controlled groups are not the only way businesses get aggregated. Affiliated service groups under IRC Section 414(m) reach the same result, but the trigger is the type of services the businesses perform and how they work together, not raw ownership.¹ A medical practice and an entity that regularly performs services with it, or a management company whose principal business is running another organization, can be aggregated even if the ownership tests above are all failed. If your setup is a professional practice sharing space, staff, or referrals with another business, controlled group testing alone won’t tell you whether you’re clear.

What Changes Once Your Businesses Are a Controlled Group

Being a controlled group doesn’t force you to run one 401(k) for everyone. Each entity can sponsor its own plan, or skip a plan entirely. What you can’t do is test any of those plans in isolation. Every employee across every member entity has to be included in the plan’s required testing, which means each plan’s recordkeeper needs employee data from the other members.

Coverage Testing

The coverage test under IRC Section 410(b) checks whether the plan covers a fair proportion of non-highly compensated employees compared with highly compensated employees. For 2026, the highly compensated employee threshold is $160,000 in prior-year compensation.⁵ If a controlled group has 200 employees across three companies and only the owners’ company has a plan, coverage testing will fail because the other 150 workers are excluded from a plan they’re deemed part of.

Nondiscrimination Testing

The nondiscrimination test under IRC Section 401(a)(4) ensures the plan doesn’t disproportionately favor highly compensated employees in the contributions or benefits actually delivered.⁶ The related ADP and ACP tests compare average deferrals and matching contributions of highly compensated employees against the rest of the workforce. Too wide a spread, and the plan fails.

Top-Heavy Testing

A plan is top-heavy under IRC Section 416 when key employees hold more than 60 percent of total plan assets. In a controlled group, all plans maintained by group members combine into a required aggregation group for this test, and if the aggregation group is top-heavy, every plan in it is.⁷ That triggers a minimum employer contribution of 3 percent of compensation for each non-key employee who participates, regardless of whether that employee defers anything themselves.

For 2026, an officer counts as a key employee if compensation exceeds $235,000.⁸ Officer status is judged at the specific entity where the person works, but compensation from every aggregated employer counts toward the dollar threshold.

Contribution Limits Are Per Person, Not Per Company

The 2026 employee deferral limit of $24,500⁹ and the $72,000 total annual additions cap⁵ apply per person across the entire group. An employee who works for two member companies can’t defer the full limit into each; the limit is shared.

One Plan or Separate Plans

A single plan across the group means one set of testing, one Form 5500, and one trust. Multiple plans mean more administrative work but allow different benefit formulas or eligibility rules for each workforce. A controlled group is a single employer for Form 5500 purposes, and the data reported is generally aggregated across all plans.¹⁰

A narrow escape route exists in IRC Section 414(r): a member can request separate line of business treatment and test on its own. The line must have at least 50 employees, serve a legitimate business purpose, and either satisfy a safe harbor for proportional representation of highly compensated employees or hold an IRS determination letter.¹ Small and mid-size businesses rarely qualify.

Mergers and Acquisitions Get a Transition Window

When a business is acquired or leaves a group, the plan doesn’t have to pass coverage testing under the new structure on day one. IRC Section 410(b)(6)(C) gives you a transition period that starts on the date of the ownership change and runs through the last day of the first plan year that begins after the change.¹¹ For a calendar-year plan where an acquisition closes in March 2026, that window runs through December 31, 2027. The plan is treated as passing coverage during the window if it passed just before the change and coverage isn’t significantly altered. Coverage has to be met by the deadline or the plan is at risk.

What It Costs to Get This Wrong

ADP or ACP test failures need to be corrected within two and a half months after the plan year ends. Miss that, and a 10 percent excise tax hits the excess amounts. Fail to correct within 12 months and the cash or deferred arrangement loses its qualified status, which can disqualify the whole plan.¹²

Late or missing Form 5500 filings draw a DOL penalty of up to $2,529 per day with no cap, plus an IRS penalty of $250 per day up to $150,000.¹³ A group that didn’t realize it needed to aggregate often files timely but wrong, which is its own problem.

Full disqualification is the worst outcome. Every dollar in the trust becomes taxable to participants, and the employer loses its prior deductions for contributions. The IRS’s Voluntary Correction Program lets sponsors fix errors before an audit for a user fee between $2,000 and $4,000 depending on plan assets.¹⁴ While a VCP submission is under review, the IRS generally won’t audit the plan.¹⁵ If you suspect your businesses have crossed into controlled group territory and your plans have been tested in isolation, VCP is generally cheaper than waiting for the IRS to find it first.

1
Office of the Law Revision Counsel. 26 USC 414 – Definitions and Special Rules
2
eCFR. 26 CFR 1.414(c)-4 – Rules for Determining Ownership
3
Office of the Law Revision Counsel. 26 USC 1563 – Definitions and Special Rules
4
Internal Revenue Service. FAQs Regarding the Aggregation Rules Under Section 448(c)(2) – Section: Q-3 What Is a Brother-Sister Controlled Group
5
Internal Revenue Service. 2026 Amounts Relating to Retirement Plans and IRAs, as Adjusted for Changes in Cost-of-Living Notice 2025-67
6
eCFR. 26 CFR 1.401(a)(4)-1 – Nondiscrimination Requirements of Section 401(a)(4)
7
eCFR. 26 CFR 1.416-1 – Questions and Answers on Top-Heavy Plans
8
Internal Revenue Service. COLA Increases for Dollar Limitations on Benefits and Contributions
9
Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026
10
Department of Labor. 2025 Instructions for Form 5500 Annual Return/Report of Employee Benefit Plan
11
Office of the Law Revision Counsel. 26 USC 410 – Minimum Participation Standards
12
Internal Revenue Service. 401(k) Plan Fix-It Guide – The Plan Failed the 401(k) ADP and ACP Nondiscrimination Tests
13
Internal Revenue Service. 401(k) Plan Fix-It Guide – You Haven’t Filed a Form 5500 This Year
14
Internal Revenue Service. Voluntary Correction Program (VCP) Fees
15
Internal Revenue Service. Voluntary Correction Program – General Description