Client concentration risk — the carrier stack-up.

When the top ten clients produce more than half of an agency's revenue, the book is no longer diversified — it's a portfolio of consulting contracts with insurance attached, and the industry label for that condition is the existential dozen. The client-side mechanics — the 15% single-client haircut, the 25% earnout trigger, the resilience levers — are the subject of the existential dozen on the customer side. This piece is about the carrier interaction: how client concentration compounds with carrier concentration, and why the overlap is the highest-severity scenario a buyer can find.

§ 01 · The consulting-contract trapWhy the multiple drops.

The 50% threshold on the top ten isn't arbitrary. At and above it, the revenue model looks more like consulting than a diversified book — and consulting firms are valued on relationships and retention, not renewal economics, so they trade at materially lower multiples. The trap is that the concentration creeps up invisibly: an agency that adds a handful of large commercial accounts over a decade can drift from 25% to 55% top-ten concentration without any strategic decision to do so. Each new large account feels like a win; the cumulative concentration is the unintended consequence, and it shows up only when a buyer applies the test. The post-close risk profile follows directly — in a diversified book losing any one client is minor, but in a consulting-contract book the probability that at least one top client exits in the first year after ownership change is meaningfully higher.

§ 02 · The two client thresholdsHaircut, then earnout.

Above 25% on a single client — or when the top three meaningfully exceed the existential-dozen threshold — the structure shifts from upfront price to a mix of upfront and earnout, conditioning some of the seller's compensation on retention through a defined period, typically 24 to 36 months. Without the earnout, the seller's incentive to help retain concentrated clients evaporates at closing; with it, the seller keeps working the relationships. Earnouts on concentrated deals usually represent 20% to 40% of total consideration, in one of two flavors: client-specific earnouts conditioned on named-account retention (cleaner legally, harder to negotiate) or revenue earnouts conditioned on aggregate book thresholds (more common, less precise).

§ 03 · The stack-upWhen the two risks correlate.

This is the reason concentration can't be read one axis at a time. A 45% top-carrier concentration looks less alarming if the top-ten clients are spread across it; a 12% top-client concentration looks healthier if the carrier portfolio is also diversified. The danger is the correlation — and sophisticated buyers increasingly fold client concentration, carrier concentration, tenure distribution, and producer distribution into a single Portfolio Stability Index precisely because the composite captures the interactions a standalone metric misses. For sellers, the index doubles as a preparation framework: improving any one component raises the score, and targeting several at once over a 24-to-36-month window compounds.

§ 04 · Building resilienceThree levers before listing.

The preparation playbook for an existential-dozen book isn't to manufacture diversity overnight — it's to show credible movement toward resilience and to document the defensibility of the concentrated relationships. Three levers work reliably. New-business diversification directs incremental production toward segments that dilute concentration without disrupting existing accounts. Relationship depth documentation formalizes the agency's hold on concentrated clients through multi-year service agreements and institutional — rather than personal — relationship maps. And producer redundancy ensures multiple producers carry substantive relationships with the top accounts, so a single producer departure doesn't endanger retention. None of the three eliminates concentration risk; all three reduce the discount it produces at exit — and a small agency that can demonstrate a declining top-ten over the two years before listing earns materially better terms than one running flat or rising.

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