The starting point is a prohibition, not a permission. Section 12(d)(1)(A) of the Investment Company Act makes it unlawful for a registered fund, and any company it controls, to acquire securities of another fund if immediately afterwards it would own more than 3 percent of the acquired fund's total outstanding voting stock, hold securities of that one fund worth more than 5 percent of its own total assets, or hold securities of investment companies generally worth more than 10 percent of its own total assets. Section 12(d)(1)(B) puts the mirror restriction on the selling side. Without relief, those three numbers make a genuine fund of funds impossible.
Why Congress restricted it says more than the numbers do. The SEC's own account is that the restrictions "are designed to prevent fund of funds arrangements that allow the acquiring fund to control the assets of the acquired fund and use those assets to enrich the acquiring fund at the expense of acquired fund shareholders", a practice the release calls pyramiding, and that Congress "also was concerned about the potential for duplicative and excessive fees when one fund invested in another and the formation of overly complex structures that could be confusing to investors". Those three concerns, control, cost and complexity, are still the shape of the current rule.
Rule 12d1-4 replaced a patchwork with a set of conditions. Adopted in 2020 and effective in January 2021, it permits fund of funds arrangements beyond the statutory limits provided the parties meet conditions on each of the three concerns. On control, the acquiring fund and its advisory group must not control an acquired fund, and above stated ownership thresholds must vote their shares in the same proportion as all other holders. On cost, the acquiring fund's adviser must evaluate the complexity and the fees and expenses of the arrangement and find that the acquiring fund's fees and expenses do not duplicate the acquired fund's, reporting that finding to the board. On complexity, the rule generally bars a third tier: a fund may not rely on it to invest above the statutory limits in a fund that is itself relying on it, unless the acquired fund keeps its own holdings of investment companies and private funds within 10 percent of its total assets. Unless the acquiring fund's own adviser also acts as the acquired fund's adviser without being a sub-adviser to either, the funds must enter a written "fund of funds investment agreement", which the rule requires to include the material terms, a termination provision on no more than 60 days' notice, and an undertaking by the acquired fund to provide fee and expense information on request. Adopting the rule, the SEC rescinded rule 12d1-2 and certain exemptive relief that had previously permitted these arrangements.
The layered-cost concern reaches the reader as a line in a fee table, and the detail is worth knowing. Form N-1A requires an open-end fund investing in acquired funds to add a subcaption directly above "Total Annual Fund Operating Expenses", titled "Acquired Fund Fees and Expenses", disclosing the fees and expenses incurred indirectly through those holdings. The instruction carries a threshold: where the indirect fees "do not exceed 0.01 percent (one basis point) of average net assets of the Fund", the fund may include them under "Other Expenses" instead. So the absence of the line does not by itself mean there is no second layer, only that any second layer was under a basis point. The instruction also sets the formula, which builds the figure from each acquired fund's own annual operating expense ratio weighted by the average balance invested and the days held, plus any transaction fees and incentive allocations.
A layered structure is not automatically an expensive one. Where the underlying funds are cheap index funds run by the same firm, and the adviser waives its own fee at the top level, the total can be lower than a comparable single-manager fund. Where the underlying funds are actively managed, or where a fee is charged at both levels for the same work, the arithmetic runs the other way. The fee table is where the question is answered, and the number to compare across funds is total annual operating expenses including the acquired fund line, not the top-level fee alone.