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County · code 17031

Cook CountyIllinois

Cook County is a county in Illinois, with its seat in Chicago. It was created on January 15, 1831, from Putnam County and named after Daniel Pope Cook.

  • Founded1831
  • County seatChicago
  • Named forDaniel Pope Cook
  • Land area945 sq mi + 690 sq mi water
  • Population5,275,541 2020 Census
  • Parcels mapped1,432,483 checked 16 October 2026
Open the free map Click any parcel in Cook County: acreage, parcel id, address and who publishes the map. No sign-in.

Updated

LakeDuPageLake, INWillPorter, INKaneMcHenryBerrien, MI Chicago Census outline · seat marked

Where it is

Cook on the map of Illinois

Illinois has 102 counties and county equivalents, each with its own records office. Shapes with a page are links.

Adams CountyAlexander CountyBond CountyBoone CountyBrown CountyBureau CountyCalhoun CountyCarroll CountyCass CountyChampaign CountyChristian CountyClark CountyClay CountyClinton CountyColes CountyCrawford CountyCumberland CountyDeKalb CountyDe Witt CountyDouglas CountyDuPage CountyEdgar CountyEdwards CountyEffingham CountyFayette CountyFord CountyFranklin CountyFulton CountyGallatin CountyGreene CountyGrundy CountyHamilton CountyHancock CountyHardin CountyHenderson CountyHenry CountyIroquois CountyJackson CountyJasper CountyJefferson CountyJersey CountyJo Daviess CountyJohnson CountyKane CountyKankakee CountyKendall CountyKnox CountyLake CountyLaSalle CountyLawrence CountyLee CountyLivingston CountyLogan CountyMcDonough CountyMcHenry CountyMcLean CountyMacon CountyMacoupin CountyMadison CountyMarion CountyMarshall CountyMason CountyMassac CountyMenard CountyMercer CountyMonroe CountyMontgomery CountyMorgan CountyMoultrie CountyOgle CountyPeoria CountyPerry CountyPiatt CountyPike CountyPope CountyPulaski CountyPutnam CountyRandolph CountyRichland CountyRock Island CountySt. Clair CountySaline CountySangamon CountySchuyler CountyScott CountyShelby CountyStark CountyStephenson CountyTazewell CountyUnion CountyVermilion CountyWabash CountyWarren CountyWashington CountyWayne CountyWhite CountyWhiteside CountyWill CountyWilliamson CountyWinnebago CountyWoodford CountyCook County Cook
Counties of Illinois · Cook highlighted

History

Formed in 1831 from Putnam County

An old deed may name a county that no longer exists, or a boundary that has since moved.

Cook County was created from Putnam County in 1831. Between 1836 and 1839, Cook County lost territory to the creation of Will County, McHenry County, and Du Page County.

Boundary changes from the Newberry Library, Atlas of Historical County Boundaries.

  1. 1831
    Cook County created Cook created from Putnam.
    from Putnam
    · Ill. Laws 1830-1831, pp. 54-57, 62-63
  2. 1836
    Lost territory Cook lost to creation of Will.
    to Will
    · Ill. Laws 1835-1836, pp. 262-263
  3. 1836
    Lost territory Cook lost to creation of McHenry. McHenry not fully organized, attached to Cook until "organized."
    to McHenry
    · Ill. Laws 1835-1836, pp. 273-278
  4. 1837
    Changed McHenry fully organized, detached from Cook.
    McHenry
    · Ill. Laws 1836-1837, pp. 89-90
  5. 1837
    Changed Legislature authorized creation of Michigan (proposed) from Cook, dependent on local referendum that failed (no change).
    Michigan
    · Ill. Laws 1836-1837, pp. 82-83
  6. 1839
    Lost territory Cook lost to creation of Du Page.
    to Du Page
    · Ill. Laws 1838-1839, pp. 73-74

Land and parcels

What the maps cover in Cook

Who publishes each layer, how good it is, and when we last checked.

Official
from the office that keeps the record
Good enough
a reliable statewide or national copy
Fallback
a guide only; check the official record
Missing
nothing public found yet

People and land

Cook by the Census

Population
5,275,541 2020 Census count
Housing units
2,264,966 2020 Census count
Land area
945 square miles
Water area
690 square miles · 42% of the county, including large bodies of water
People per sq mi
5,583 population ÷ land area

Source: US Census Bureau, 2020 Census; US Census Bureau, 2020 Gazetteer Files. Not endorsed or certified by the Census Bureau.

Rules and offices

Who to ask before you build, dig or sell

Official county pages only. We link to them; we do not copy them, because they change.

Inside a city or town, permits usually come from that city rather than the county.

Records

Permits

Zoning and code

GIS and maps

County government

See your own lot on these maps

Open Cook County on Parcel Chart's free map and click your parcel. Save it with a free account; the full record of your place (site map, projects, documents) is Pro, in early access.

Open the free map no sign-in Pro early access

Questions

Common questions about Cook County

When was Cook County founded?
Cook County was founded in 1831, formed from Putnam County, and named after Daniel Pope Cook.
What is the county seat of Cook County?
The county seat of Cook County is Chicago.
Is there an online parcel map for Cook County?
Yes. Cook County publishes a parcel map of 1,432,483 parcels, rated good enough here. You can click any parcel on it in Parcel Chart's free map, no sign-in needed.
Has Cook County's boundary or name changed?
Cook County was created in 1831 from Putnam County. Since then the Atlas of Historical County Boundaries records 4 later changes, chiefly involving McHenry County, Du Page County and Will County.
Who issues building permits in Cook County?
The county's page on building permits: Permits. Inside a city or town, ask that city first.
Is my lot in a flood zone?
We do not have flood maps for Cook County yet. Look up the address on FEMA's Flood Map Service Center, which shows the effective flood map for any lot.

Nearby

Counties next door