Wells, Septic & Rural Utilities in Sonoma County

Many rural homes in Sonoma County rely on a private well and a septic system instead of city water and sewer, and both need their own inspections before you buy: a well yield and water-quality test, and a septic inspection that confirms the system’s size, condition and permit history.

Permit Sonoma regulates wells and septic systems in unincorporated areas. Some systems carry ongoing monitoring permits, and capacity often limits ADUs, bedrooms and pools. Here’s how each works and what to check.

First, find out what the property uses

Inside the City of Sonoma, the city supplies water and the Sonoma Valley County Sanitation District handles sewer (City of Sonoma; Sonoma Water). The Valley of the Moon Water District serves much of the valley between Glen Ellen and the city (Sonoma County Civil Grand Jury). Beyond those areas, wells and septic are common.

Wells

How the county regulates them

• Permits: new wells and well destruction need a permit from Permit Sonoma, with a site plan and supporting documents under County Code chapter 25B (Permit Sonoma: well permitting).
• Metering: new water wells must be metered, except residential wells serving a single parcel with total groundwater use of 2.0 acre-feet a year or less.
• Stream review: new supply wells in the county’s Public Trust Review Area are reviewed for effects on nearby streams and rivers.
• Yield testing: dry-weather yield tests must be run by a licensed professional; property owners and real estate agents aren’t authorized to conduct them.

Groundwater basins matter too. The Sonoma Valley subbasin is rated medium priority under the state’s groundwater law (California DWR), and the state approved sustainability plans for the Sonoma Valley, Santa Rosa Plain and Petaluma Valley basins in January 2023 (California Natural Resources Agency). Rural well owners in those basins pay a groundwater fee through their tax bill, estimated in 2023 at about $20 a year for a typical home (KRCB).

What to check before you buy

1. The well log: depth, construction and date, from the county or the seller.
2. A flow or yield test by a licensed well professional, ideally in late summer or fall when yields are lowest.
3. Water quality: bacteria, nitrates, arsenic and other local concerns; ask the tester what’s common in the area.
4. Equipment: pump age, pressure tank, storage tanks and any treatment system.
5. Shared wells: if a well serves more than one home, get the written well-sharing agreement.

If you plan to add an ADU, the county may require a dry-weather well test between July 15 and October 1 in water-scarce areas, and wells in its lowest groundwater-availability class must meet a net-zero groundwater use policy (Permit Sonoma: ADUs).

Septic systems

How the county regulates them

• Design and construction: new, replacement and repaired systems need an OWTS construction permit under the county’s OWTS Manual. A separate design application can show that a property can support a system, for example before a land purchase or lot line adjustment (Permit Sonoma: OWTS applications).
• Site evaluations: soil profiles, percolation tests and wet-weather groundwater determinations are done by septic professionals under a county permit, with county staff observing by video (Permit Sonoma: site evaluations).
• Operational permits: systems with supplemental treatment need an operational permit and monitoring. Owners self-monitor in March and October, a service provider maintains the pretreatment unit for the life of the system, and an easement allowing county inspections is recorded on the property (Permit Sonoma: OPR program).

What to check before you buy

1. The permit record: what was approved, for how many bedrooms, and whether there’s a reserve area.
2. An inspection by a qualified septic inspector, including pumping the tank and checking the leach field.
3. Operational permit status for alternative systems: annual fees, monitoring reports and the service contract transfer to you.
4. Location: where the tank and leach field are, so you don’t plan a pool, ADU or driveway on top of them.
5. Bedroom count vs permit: a home with more bedrooms than its septic permit allows is a red flag.

How wells and septic limit what you can build

Septic capacity is often the real limit on a rural lot. Adding bedrooms or an ADU may require checking the existing system’s capacity, a “bedroom swap” or a new system, and the county wants any sewer or septic permit issued before you apply for the building permit (Permit Sonoma). Wells may need a test showing enough water. Our guide to land, building lots and ADUs explains the process.

Where you’ll find wells and septic

Expect them on hillside and rural properties around Glen Ellen and Kenwood, on Sonoma Mountain, in west county, along the Russian River and on most vineyard, ranch and acreage parcels. If vines are part of your plan, read buying a small hobby vineyard and our guide to Sonoma County vineyards.

Buying rural for the first time? Our guide to buying a home in Sonoma County covers the full inspection list, and you can contact us to talk through a specific property.

Questions people ask

Can I get a loan on a home with a well and septic?

Yes. Lenders often ask for water tests and sometimes a septic inspection; ask yours what it requires.

How often should a septic tank be pumped?

It depends on tank size and use. Your inspector can recommend a schedule; alternative systems follow their operational permit.

What does a failing septic system cost to fix?

It varies with soil, site and system type. Get a design from a septic professional before you price it into an offer.

Sources & notes

• Permit Sonoma: well permitting; OPR program; Septic site evaluations; OWTS application types; ADU information
• DWR: Sonoma Valley subbasin; GSP approvals; KRCB: groundwater fee
• City of Sonoma: utilities; Sonoma Water: SVCSD; Civil Grand Jury: Sonoma Valley water; Sonoma Clean Power

Rules change; confirm current requirements with Permit Sonoma. Last reviewed October 2026 by Matthew Martinez.