When was Contra Costa County founded?

Contra Costa County was founded in 1850, and named after opposite coast.

What is the county seat of Contra Costa County?

The county seat of Contra Costa County is Martinez.

Is there an online parcel map for Contra Costa County?

Yes. A public source publishes a parcel map of 316,171 parcels, rated good enough here. You can click any parcel on it in Parcel Chart's free map, no sign-in needed.

Has Contra Costa County's boundary or name changed?

Contra Costa County was created in 1850. Since then the Atlas of Historical County Boundaries records 6 later changes, most of them small boundary adjustments, chiefly involving Alameda County, Marin County and San Joaquin County.

Is my lot in a flood zone?

We do not have flood maps for Contra Costa County yet. Look up the address on FEMA's Flood Map Service Center, which shows the effective flood map for any lot.

County · code 06013

Contra Costa CountyCalifornia

Contra Costa County is a California county with its seat in Martinez, formed on February 18, 1850.

Founded1850
County seatMartinez
Named foropposite coast
Land area717 sq mi + 87 sq mi water
Population1,165,927 2020 Census
Parcels mapped316,171 checked 15 October 2026

Open the free map Click any parcel in Contra Costa County: acreage, parcel id, address and who publishes the map. No sign-in.

Updated 19 October 2026

Census outline · seat marked

History

Founded in 1850

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

Contra Costa County was created from non-county area on February 18, 1850, and named after the opposite coast. The county lost territory to San Joaquin County in 1852 and to Alameda and Santa Clara counties in 1853, but gained the Brothers Islands from Marin County in 1868.

Boundary changes from the Newberry Library, Atlas of Historical County Boundaries; the 5 that matter most are shown first.

1850
Contra Costa County created Contra Costa created from non-county area. 18 February 1850 · Calif. Stats. 1849, 1st sess., ch. 15, sec. 10/p. 60
1852
Lost territory Contra Costa lost to San Joaquin.
to San Joaquin
14 February 1852 · Calif. Stats. 1852, 3d sess., chs. 105-106/pp. 178-180; Purcell, 250
1853
Lost territory Contra Costa lost to creation of Alameda, and lost small remnant to Santa Clara.
to Alameda to Santa Clara
25 March 1853 · Calif. Stats. 1853, 4th sess., ch. 41/pp. 56-59
1868
Gained territory Contra Costa gained the Brothers Islands from Marin.
from Marin
26 March 1868 · Calif. Stats. 1867, 17th sess., ch. 305/p. 347
1958
Lost territory Contra Costa lost small area to Alameda.
to Alameda
14 January 1958 · Coy, 338-339

Show all 9 changes (4 more, mostly small adjustments)

1850
Contra Costa County created Contra Costa created from non-county area. 18 February 1850 · Calif. Stats. 1849, 1st sess., ch. 15, sec. 10/p. 60
1850
Boundary clarified Contra Costa boundaries clarified (no change).
no change
5 April 1850 · Calif. Stats. 1849, 1st sess., ch. 61, sec. 4/p. 156
1851
Boundary clarified Contra Costa boundaries redefined (no change).
no change
25 April 1851 · Calif. Stats. 1851, 2d sess., ch. 14, sec. 10/p. 174
1852
Lost territory Contra Costa lost to San Joaquin.
to San Joaquin
14 February 1852 · Calif. Stats. 1852, 3d sess., chs. 105-106/pp. 178-180; Purcell, 250
1853
Lost territory Contra Costa lost to creation of Alameda, and lost small remnant to Santa Clara.
to Alameda to Santa Clara
25 March 1853 · Calif. Stats. 1853, 4th sess., ch. 41/pp. 56-59
1868
Gained territory Contra Costa gained the Brothers Islands from Marin.
from Marin
26 March 1868 · Calif. Stats. 1867, 17th sess., ch. 305/p. 347
1958
Lost territory Contra Costa lost small area to Alameda.
to Alameda
14 January 1958 · Coy, 338-339
1992
Lost territory Contra Costa lost small area to Alameda. Ordinance passed 11 August 1992; effective 1 September 1992.
to Alameda
1 September 1992 · Minutes of the Board of Supervisors, Alameda County, 11 August 1992, ordinance 92-57
1995
Gained territory Contra Costa gained small area from Alameda.
from Alameda
4 October 1995 · Calif. Stats. 1995, reg. sess., ch. 529, sec. 3 in Official California Legislative Information, http://www.leginfo.ca.gov/statute.html, 23 September 2002

Land and parcels

What the maps cover in Contra Costa

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

Parcels Good enough

Lot lines and tax-map numbers for 316,171 parcels. Drawn for tax purposes: usually within a few feet of a survey, but not a survey.
checked 15 October 2026
Address points Missing

No public address points layer found for Contra Costa County yet.
Buildings Reference

Roof outlines traced from aerial imagery from FEMA Response Geospatial Office with Oak Ridge National Laboratory (ORNL). Good for seeing what is on a lot; not accurate enough to set a fence from.
checked 18 October 2026

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

Contra Costa by the Census

Population

1,165,927 2020 Census count

Housing units

423,342 2020 Census count

Land area

717 square miles

Water area

87 square miles

People per sq mi

1,626 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

Contra Costa County's official site

We have found the county's site but not its offices yet; start there.

County government

County government
The official website of Contra Costa County.
contracosta.ca.gov

See your own lot on these maps

Open Contra Costa 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 Contra Costa County

When was Contra Costa County founded?: Contra Costa County was founded in 1850, and named after opposite coast.
What is the county seat of Contra Costa County?: The county seat of Contra Costa County is Martinez.
Is there an online parcel map for Contra Costa County?: Yes. A public source publishes a parcel map of 316,171 parcels, rated good enough here. You can click any parcel on it in Parcel Chart's free map, no sign-in needed.
Has Contra Costa County's boundary or name changed?: Contra Costa County was created in 1850. Since then the Atlas of Historical County Boundaries records 6 later changes, most of them small boundary adjustments, chiefly involving Alameda County, Marin County and San Joaquin County.
Is my lot in a flood zone?: We do not have flood maps for Contra Costa County yet. Look up the address on FEMA's Flood Map Service Center, which shows the effective flood map for any lot.

Nearby