When was Sonoma County founded?

Sonoma County was founded in 1850, formed from Marin County and Mendocino County, and named after Sonoma, California.

What is the county seat of Sonoma County?

The county seat of Sonoma County is Santa Rosa.

Is there an online parcel map for Sonoma County?

Yes. Sonoma County publishes a parcel map of 188,769 parcels, rated good enough here. You can click any parcel on it in Parcel Chart's free map, no sign-in needed.

Has Sonoma County's boundary or name changed?

Sonoma County was created in 1850 from Marin County and Mendocino County. Since then the Atlas of Historical County Boundaries records 9 later changes, chiefly involving Mendocino County, Marin County and Solano County.

Who issues building permits in Sonoma County?

The county's page on building permits: Permit. Inside a city or town, ask that city first.

Who issues septic and well permits in Sonoma County?

The county's page on septic systems and wells: Health department: septic and well.

Is my lot in a flood zone?

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

County · code 06097

Sonoma CountyCalifornia

Sonoma County is a county in California, with its seat in Santa Rosa. It was created on February 18, 1850.

Founded1850
County seatSanta Rosa
Named forSonoma, California
Land area1,576 sq mi + 192 sq mi water
Population488,863 2020 Census
Parcels mapped188,769 checked 15 October 2026

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

Updated 19 October 2026

Census outline · seat marked

History

Formed in 1850 from Marin County and Mendocino County

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

Sonoma County was created from non-county area on February 18, 1850, with Marin and Mendocino counties temporarily attached for judicial purposes. Marin was fully organized and detached in April 1850, while Mendocino remained attached until it was fully organized in May 1859. The county experienced several boundary adjustments with neighboring counties, including exchanges with Mendocino in 1859 and 1917, and transfers with Marin, Napa, and Solano.

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

1850
Sonoma County created Sonoma created from non-county area. Marin and Mendocino not fully organized, attached to Sonoma "for judicial purposes."
from Marin from Mendocino
18 February 1850 · Calif. Stats. 1849, 1st sess., ch. 15, sec. 12/p. 60
1852
Lost territory Sonoma lost Mare Island when Sonoma boundaries were redefined. Mare Island remained a non-county area until 27 January 1853 when it became part of Solano.
to Solano
3 May 1852 · Calif. Stats. 1852, 3d sess., ch. 147/p. 236
1855
Lost territory Sonoma lost to Napa.
to Napa
4 April 1855 · Calif. Stats. 1855, 6th sess., ch. 68/p. 77
1855
Gained territory Sonoma gained from Marin, exchanged with Mendocino, lost to Solano.
from Marin from Mendocino from Solano
27 April 1855 · Calif. Stats. 1855, 6th sess., ch. 125/p. 150
1859
Exchanged territory Sonoma exchanged with Mendocino.
with Mendocino
11 March 1859 · Calif. Stats. 1859, 10th sess., ch. 108/pp. 98-99

Show all 12 changes (7 more, mostly small adjustments)

1850
Sonoma County created Sonoma created from non-county area. Marin and Mendocino not fully organized, attached to Sonoma "for judicial purposes."
from Marin from Mendocino
18 February 1850 · Calif. Stats. 1849, 1st sess., ch. 15, sec. 12/p. 60
1850
Changed Marin fully organized, detached from Sonoma. Mendocino remained attached to Sonoma.
Marin Mendocino
22 April 1850 · Calif. Stats. 1849, 1st sess., ch. 131/p. 406
1851
Boundary clarified Sonoma boundaries redefined (no change).
no change
25 April 1851 · Calif. Stats. 1851, 2d sess., ch. 14, sec. 24/p. 178
1852
Lost territory Sonoma lost Mare Island when Sonoma boundaries were redefined. Mare Island remained a non-county area until 27 January 1853 when it became part of Solano.
to Solano
3 May 1852 · Calif. Stats. 1852, 3d sess., ch. 147/p. 236
1855
Lost territory Sonoma lost to Napa.
to Napa
4 April 1855 · Calif. Stats. 1855, 6th sess., ch. 68/p. 77
1855
Gained territory Sonoma gained from Marin, exchanged with Mendocino, lost to Solano.
from Marin from Mendocino from Solano
27 April 1855 · Calif. Stats. 1855, 6th sess., ch. 125/p. 150
1859
Exchanged territory Sonoma exchanged with Mendocino.
with Mendocino
11 March 1859 · Calif. Stats. 1859, 10th sess., ch. 108/pp. 98-99
1859
Changed Mendocino fully organized, detached from Sonoma.
Mendocino
2 May 1859 · Calif. Stats. 1859, 10th sess., ch. 108, sec. 2/p. 99; History of Mendocino County, 272-273, 283
1860
Lost territory Sonoma lost to Marin when boundary was changed to run on road.
to Marin
25 April 1860 · Calif. Stats. 1860, 11th sess., ch. 288/p. 269
1861
Gained territory Sonoma gained from Marin (later repealed).
from Marin
13 May 1861 · Calif. Stats. 1861, 12th sess., ch. 346/p. 351
1868
Boundary clarified Sonoma boundaries redefined (no change).
no change
10 February 1868 · Calif. Stats. 1867, 17th sess., ch. 45/p. 42
1917
Exchanged territory Sonoma exchanged with Mendocino.
with Mendocino
30 July 1917 · Calif. Stats. 1917, 42d sess., ch. 725/p. 1396

Land and parcels

What the maps cover in Sonoma

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

Parcels Good enough

Lot lines and tax-map numbers for 188,769 parcels from Sonoma County. Drawn for tax purposes: usually within a few feet of a survey, but not a survey.
edited 11 December 2024 · checked 15 October 2026
Address points Missing

No public address points layer found for Sonoma 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

Sonoma by the Census

Population

488,863 2020 Census count

Housing units

204,742 2020 Census count

Land area

1,576 square miles

Water area

192 square miles

People per sq mi

310 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

Assessor and property records 707-565-2281from sonomacounty.gov · checked 13 October 2026

Permits

Health department: septic and well 707-565-6565from sonomacounty.gov · checked 13 October 2026
Well Water Qualityfrom sonomacounty.gov
Permitfrom permitsonoma.org
Permitsfrom prmd.sonomacounty.ca.gov

Zoning and code

County with intercounty ordinance (13 counties)from boe.ca.gov
County Municipal Codefrom library.municode.com

County government

County government
The official website of Sonoma County.
sonomacounty.gov

See your own lot on these maps

Open Sonoma 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 Sonoma County

When was Sonoma County founded?: Sonoma County was founded in 1850, formed from Marin County and Mendocino County, and named after Sonoma, California.
What is the county seat of Sonoma County?: The county seat of Sonoma County is Santa Rosa.
Is there an online parcel map for Sonoma County?: Yes. Sonoma County publishes a parcel map of 188,769 parcels, rated good enough here. You can click any parcel on it in Parcel Chart's free map, no sign-in needed.
Has Sonoma County's boundary or name changed?: Sonoma County was created in 1850 from Marin County and Mendocino County. Since then the Atlas of Historical County Boundaries records 9 later changes, chiefly involving Mendocino County, Marin County and Solano County.
Who issues building permits in Sonoma County?: The county's page on building permits: Permit. Inside a city or town, ask that city first.
Who issues septic and well permits in Sonoma County?: The county's page on septic systems and wells: Health department: septic and well.
Is my lot in a flood zone?: We do not have flood maps for Sonoma County yet. Look up the address on FEMA's Flood Map Service Center, which shows the effective flood map for any lot.

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