Yes, in the sense that matters: the county your business address sits in decides which of Illinois’ 13 rating areas your group is rated in, and rating area is one of the inputs a small-group premium is legally permitted to vary on. No, in the sense people usually mean: nothing in the research behind this page says which Illinois areas are dearer or cheaper, and this page will not invent a ranking.
What is worth knowing is the boundary itself, because Illinois draws it in a way that catches employers out. The state’s largest metropolitan market is split across several rating areas, and the line that splits it is a county line.
Illinois draws its rating areas on county lines and nothing else
Federal rules give a state a choice. 45 CFR 147.102(b)(3) opens: “A state’s rating areas must be based on the following geographic boundaries: Counties, three-digit zip codes, or metropolitan statistical areas and non-metropolitan statistical areas, as defined by the Office of Management and Budget…”
Illinois took counties. The CMS table publishing the Illinois areas for the individual and small group market carries 102 county rows — matching the state’s 102 counties exactly — and every county is assigned to exactly one of 13 areas. The table has a column headed for a three-digit zip code, and it is empty on all 102 rows.
That single structural fact removes a whole class of confusion. A zip code does not place an Illinois small group in a rating area, because Illinois never populated the column that would let it. Neither does a metro name. The unit is the county.
The address that decides it belongs to the business, not the employee
The determinant is federal, and it is specific. 45 CFR 147.102(a)(1)(ii)(B) reads:
“In the small group market, using the group policyholder’s principal business address. For purposes of this paragraph (a)(1)(ii)(B), principal business address means the principal business address registered with the State or, if a principal business address is not registered with the State, or is registered solely for purposes of service of process and is not a substantial worksite for the policyholder’s business, the business address within the State where the greatest number of employees of such policyholder works. If, for a network plan, the group policyholder’s principal business address is not within the service area of such plan, and the policyholder has employees who live, reside, or work within the service area, the principal business address for purposes of the network plan is the business address within the plan’s service area where the greatest number of employees work as of the beginning of the plan year. If there is no such business address, the rating area for purposes of the network plan is the rating area that reflects where the greatest number of employees within the plan’s service area live or reside as of the beginning of the plan year.”
Read the cascade rather than the first clause. The default is the registered principal business address. A registered address that exists only to accept service of process and is not a substantial worksite does not count, and the rule falls through to the Illinois business address where the greatest number of employees works. Employee residence appears only at the bottom of a network-plan fallback, and only after the principal business address has failed to sit inside the plan’s service area.
For an ordinary small employer with one Illinois location, all of that collapses to a single question: which county is the building in.
Cook County is a rating area by itself, and the Chicago region is not
Illinois has one county that forms an entire rating area on its own, and it is Cook. Chicago sits in Cook, so a Chicago business address means Area 1, alone.
The surrounding metropolitan counties do not join it. Lake and McHenry sit together in Area 2. DuPage and Kane sit together in Area 3. Grundy, Kankakee, Kendall and Will sit together in Area 4.
Illinois law, meanwhile, treats that whole territory as one place. The Regional Planning Act defines the term: “‘Region’ or ‘northeastern Illinois region’ means Cook, DuPage, Kane, Kendall, Lake, McHenry, and Will Counties.” Those are the counties the State itself groups as a single metropolitan region, and the federal rating table spreads them across several different areas.
So the collar-county move that feels like nothing — an office relocation from Chicago to a suburb a short drive away — can change the rating area, while a move of the whole workforce’s homes changes nothing at all. Geography for rating purposes is the county line under the business address, not the labor market.
Joliet sits in the one area that straddles the region line
Will County is the clearest illustration in the state, and Joliet sits in it.
Will shares Area 4 with Kendall, Grundy and Kankakee. Kendall and Will are inside the statutory northeastern Illinois region; Grundy and Kankakee are not. That makes Area 4 the only Illinois rating area that mixes counties the State places inside its statutory Chicago region with counties it places outside it.
The practical version matters most to warehousing and logistics employers, where the building is in Will County and a large share of the staff commute in from Cook or DuPage. The rating geography follows the building. A Joliet employer is rated alongside Grundy and Kankakee rather than alongside its DuPage or Cook neighbors, whatever the commute map looks like.
Metro East is where a statutory boundary and a rating-area boundary actually coincide
“Metro East” is not a federal term and it does not appear on the CMS table. The Illinois side of the St. Louis metropolitan area is Madison, St. Clair and Monroe counties, and the table places all three in Area 12 together.
That is the one Illinois market where a statutory district boundary and a rating-area boundary line up exactly. The Bi-State Development Compact describes its district as embracing “The City of St. Louis and the counties of St. Louis and St. Charles and Jefferson in Missouri, and the counties of Madison, St. Clair, and Monroe in Illinois.” The three Illinois counties in that list are precisely the membership of Area 12. The separately drawn Southwestern Illinois Development Authority, which adds Bond and Clinton, does not line up the same way.
The interesting complication on this side of the state is the state line rather than the county line. An employer anywhere on the Illinois bank carries the same rating geography while hiring from a labor market that runs into Missouri, where a different insurance code governs. An Illinois-sited group carries Illinois rules for every enrolled employee, including the ones who live on the Missouri side.
Downstate, the same county line does different work in each market
The county rule is uniform. What it produces is not.
Peoria County shares Area 7 with Tazewell and Woodford — the other counties the Illinois River metro is built across. The river that divides the metro between counties therefore does not divide it between rating areas, and employers on either bank carry the same rating geography. The statutory Tri-County River Valley Development Authority reaches one county further, into McLean, and McLean does sit in a different area.
Winnebago County, where Rockford is the county seat, sits in Area 5. The General Assembly placed Winnebago inside the territorial jurisdiction of the Quad Cities Regional Economic Development Authority, whose statute names “Jo Daviess, Carroll, Whiteside, Stephenson, Lee, Rock Island, Henry, Knox, Mercer, Winnebago, and Boone counties in the State of Illinois.” The rating table does not follow that line — those counties are split across separate areas, so a Winnebago employer and a Rock Island employer inside the same statutory economic district do not share rating geography. Area 5 is a northern-tier band containing no county of the statutory northeastern Illinois region at all.
Sangamon County, the site of one of the two offices the Illinois Department of Insurance keeps, is in Area 10. It is the only downstate market where the regulator that reviews health insurance filings and handles coverage complaints has an office in the same county. That area is a broad central band running west toward the Mississippi and taking in another downstate metro’s core county, and the statutory Central Illinois Economic Development Authority that includes Sangamon is itself split across separate rating areas. A Springfield business address is pooled across a largely rural central band rather than rated on the Springfield market alone.
Jackson County, where Carbondale sits, is in Area 13 — the largest Illinois rating area by county count, a single area covering most of the southern end of the state. Every county named in the Southern Illinois Economic Development Authority Act sits inside it except Randolph, which is the reminder that even a wide area has an edge somewhere.
The Illinois rating act a search engine will hand you, and why it does not answer this
Search for how Illinois regulates small-group premiums and you will find the Small Employer Health Insurance Rating Act, 215 ILCS 93, with its class-of-business machinery, its index rate, and its corridors around that index rate. All of it is still printed, still readable, and still returned by search engines.
Read one section earlier than the mechanism, though, and the Act switches itself off. 215 ILCS 93/15(b):
“This Act shall not apply to any health benefit plan for a small employer that is delivered, issued, renewed, or continued in this State on or after January 1, 2022. However, if 42 U.S.C. 18032(c)(2) or any successor law is repealed, then this Act shall apply to each health benefit plan for a small employer that is delivered, issued, renewed, or continued in this State on or after the date that law ceases to apply to such plans.”
The revival condition was checked rather than assumed: 42 U.S.C. 18032(c)(2), the single-risk-pool rule for the small group market, is in force and has not been repealed. So the condition is untriggered and the Act stays disapplied.
This is a genuine trap rather than a technicality. Section 25 of that Act was amended in 2023 — after it had stopped applying to new and renewing plans. A writer can quote it accurately, cite it perfectly, and still describe something that governs no ACA-compliant Illinois small-group plan issued since the disapplication date. It is also internally in tension: Section 10 of the same Act excludes claim experience, health status and duration of coverage from “case characteristics,” while Section 25 admits a bounded renewal adjustment made expressly on account of them. Both texts are reported here and neither is resolved, and nothing on this page asserts that federal law overrides the Illinois Act.
For a county question, the upshot is simple: the county-to-area assignment does not come from that Act. It comes from the federal rating rule and the CMS table.
What the federal table does not say, including two county names
Two cautions travel with the source.
The table carries no effective plan year. The full page was read before and after the table, along with every date-like string in it, and there is no “applicable for plan year” language and no revision history. The only date on it is its own on-page modification stamp, “Page Last Modified: 09/10/2024 06:11 PM.” The page also returns a much fresher timestamp in its transport metadata, and that is a content-delivery artifact rather than a publication date — taking it at face value would make an older table look freshly revised. Nothing here is year-bounded, because the source does not support a year-bounded claim.
And the table misspells two Illinois county names. Illinois spells them DuPage and Vermilion. The federal table prints each differently, which is a trivia point for a reader and a real problem for anyone joining county data against that table by name — an unnormalized join drops a county silently, and the dropped county is one of the ones you would most want to be right about.
What none of this tells you about what you will pay
Rating area is one of the inputs a small-group rate is permitted to vary on, and Illinois defines those areas by county. That is the whole of what the sources establish about geography and price.
They do not establish direction. No premium figure by Illinois rating area appears in any source used for this brand’s Illinois work, so there is no honest way to say that Cook is dearer than DuPage, or that the southern band is cheaper than the collar counties, or that crossing a line saves anything. An employer told otherwise is being told something the public record does not contain.
What the geography is genuinely useful for is narrower. It tells you that the address on the application is a rating decision, that the registered address and the real worksite can be different places and the rule knows it, and that in the Chicago market a decision about which suburb to lease in is also a decision about which rating area your group lands in.
The bottom line
County is a real input: Illinois builds all 13 of its rating areas out of whole counties, and the group policyholder’s principal business address decides which one applies. Direction is a different question — no source we rely on publishes a premium by Illinois rating area, so this page explains the boundary and will not rank it.
Frequently asked questions
Which county decides my group’s rating area — where the business is, or where employees live?
The business. 45 CFR 147.102(a)(1)(ii)(B) sets the small-group rating area using the group policyholder’s principal business address, which the rule defines as the principal business address registered with the State, or, where none is registered or the registered one is only a service-of-process address and not a substantial worksite, the business address within the State where the greatest number of employees works. Employee residence enters only through a narrow network-plan fallback the rule spells out, and only where the principal business address falls outside the plan’s service area. For an ordinary Illinois small group, the county on the business address is the county that matters.
How many rating areas does Illinois have, and what are they built from?
Illinois has 13. They are built from whole counties and nothing else. The CMS table for Illinois carries 102 county rows, matching the state’s 102 counties exactly, and assigns every county to exactly one area. The table also carries a column headed for a three-digit zip code, and that column is empty on all 102 rows. Federal rules at 45 CFR 147.102(b)(3) let a state draw rating areas on counties, three-digit zip codes, or metropolitan and non-metropolitan statistical areas; Illinois chose counties, which is why the zip column was never filled in.
Is one Illinois rating area cheaper than another?
Nothing in the research behind this page answers that, and we will not guess at it. No premium figure by Illinois rating area appears in any source used for this brand’s Illinois work. What is established is structural: rating area is one of the inputs a small-group rate is permitted to vary on, and Illinois defines those areas by county. Direction and magnitude are a separate claim that would need a source none of this rests on, so no Illinois county or market is described here as more or less expensive than another.
My office is in Chicago but half my staff commute in from the collar counties. Does that split my rating?
No. The rating area follows the group policyholder’s principal business address, so a single Illinois business address puts the whole group in one area regardless of where employees commute from. The split that does matter is between business addresses: Cook County is a rating area by itself, while Lake and McHenry sit together in another, DuPage and Kane in another, and Grundy, Kankakee, Kendall and Will in another. Moving an office across the Cook County line changes the area without leaving the metropolitan region the State itself defines as one.
Do the Illinois rating areas apply to a particular plan year?
That cannot be affirmed from the source, and asserting it would be inventing a date. The CMS page publishing the Illinois areas carries no effective-plan-year statement, no applicable-year language, and no revision history. The entire page was read before and after the table, along with every date-like string in it, and the only date it carries is its own on-page modification stamp. The page’s transport metadata reports a much more recent date, but that is a content-delivery timestamp rather than a publication date and it was discarded. Treat the areas as published without a stated plan-year scope.
About the author
Nate Jones, CPCU, is the founder of Wexford Insurance and Wexford Health Insurance, an independent agency placing small-business group benefits in 9 states. He works through Illinois rating-area geography with small employers whose business address, worksites and workforce sit in different counties of the same metropolitan market. Start with the Wexford Health Insurance quote form.