When a business owner controls multiple entities, the IRS treats them as a single employer for certain tax purposes. The controlled group rules under IRC §1563 prevent business owners from multiplying tax benefits - like retirement plan contribution limits, the §179 expensing limit, or the small business health insurance credit - by spreading operations across artificially separate entities. Understanding whether your entities form a controlled group is essential before setting contribution limits, claiming small business credits, or making entity-level elections that have per-business caps.
Parent-subsidiary controlled group: One corporation (the parent) owns at least 80% of the stock of one or more other corporations (subsidiaries). The most common structure: a holding company that owns operating subsidiaries.
Brother-sister controlled group: Two or more corporations in which the same five or fewer persons own: (a) at least 80% of each corporation, AND (b) more than 50% of each corporation taking into account only the ownership that is identical across all corporations. Both tests must be met simultaneously.
Combined group: Three or more corporations where each corporation is a member of either a parent-subsidiary or brother-sister group, and at least one corporation is the common parent of a parent-subsidiary group and also a member of a brother-sister group.
When entities are in a controlled group, they are treated as a single employer for the following purposes - meaning limits and thresholds that apply per-employer apply to the group as a whole:
Retirement plan nondiscrimination and contribution limits: Under IRC §414(b) and §414(c), all employees of a controlled group are treated as employed by a single employer. This means: (1) highly compensated employees across all entities are counted together for nondiscrimination testing; (2) the 401(k) $23,500 employee deferral limit applies per individual across all controlled group entities; (3) the §415 total contribution limit of $70,000 applies per individual, not per entity. A business owner who tries to maximize contributions to separate 401(k) plans at two controlled group entities will be capped at the §415 limit across both.
§179 expensing limit: The §179 $2,500,000 limit (2026) applies to the controlled group as a whole. If two brother-sister corporations each try to claim $2,500,000 of §179 expensing, the combined deduction is still limited to $2,500,000 total, allocated among the members.
The brother-sister test is the most commonly misapplied controlled group analysis. Both tests must be satisfied simultaneously:
80% test: The same five or fewer individuals (counting all ownership together) must own at least 80% of each corporation. Example: Individual A owns 60% of Corp X and 70% of Corp Y. Individual B owns 30% of Corp X and 20% of Corp Y. Together A+B own 90% of X and 90% of Y - the 80% test is met for both.
50% identical ownership test: Count only the ownership that is identical across all corporations. In the example: A owns 60% of X and 70% of Y - identical ownership is 60%. B owns 30% of X and 20% of Y - identical ownership is 20%. Total identical: 80%. Since 80% exceeds 50%, the 50% test is met. Both tests pass - this is a brother-sister controlled group.
Ownership is determined using attribution rules that treat certain related party ownership as constructive ownership. Key attributions: stock owned by a corporation is attributed to its shareholders proportionally; stock owned by a partnership is attributed to its partners proportionally; stock owned by a spouse is attributed to the other spouse; stock owned by minor children is attributed to parents. These attribution rules can create controlled group membership that is not obvious from direct ownership alone.