Wells, Septic and Groundwater in Napa County: A Buyer's Guide
Outside city water and sewer service, most Napa County homes depend on a private well and an onsite wastewater (septic) system, and both are permitted by the Environmental Health Division of Napa County Planning, Building & Environmental Services (PBES). A new or replacement well needs a county well permit, a septic system is sized from a site evaluation by a qualified professional, and a home that pumps its own groundwater inside the Napa Valley Subbasin now pays a groundwater sustainability fee on the property tax bill: $28.17 per parcel for fiscal year 2026-27.
This guide is for buyers of rural homes, estates and small acreage in places like Pope Valley, Angwin and the hills around the valley. It explains who regulates what, which records to pull before you remove contingencies, and how the groundwater plan affects a homeowner. Vineyard and winery water rules, including the Water Availability Analysis, are covered in our companion guide to Napa County groundwater rules.
Who regulates wells, septic and groundwater
| Agency | What it handles | When a buyer meets it |
|---|---|---|
| PBES Environmental Health, Well and Onsite Wastewater Treatment | Permits for water wells, monitoring wells, well destruction and reconstruction; conventional, engineered and alternative septic systems, holding tanks and sewer lines; site evaluations and soil borings | Any new well, septic repair or replacement, bedroom addition or new dwelling |
| PBES Planning | Use permits and other discretionary projects, including any Water Availability Analysis | Wineries, larger agricultural projects, some second-unit or commercial uses |
| Napa County Groundwater Sustainability Agency (NCGSA) | The Napa Valley Subbasin groundwater plan and the groundwater sustainability fee | Parcels inside the subbasin that use groundwater |
| California Department of Water Resources (DWR) | Well completion reports filed by drillers | When you look up how and when a well was built |
| State Water Resources Control Board | The statewide septic (OWTS) policy and drinking water systems; Napa County oversees systems with fewer than 200 connections under a state contract | Shared wells serving five or more connections, and wineries that serve water to visitors |
Environmental Health covers unincorporated Napa County and parcels inside city limits that aren’t on public sewer. Its office is at 1195 Third Street, 2nd Floor, in Napa (707-253-4417), and inspection requests go to 707-253-4135.
Checking an existing well before you buy
A listing that says “private well” tells you very little. These are the records and tests that answer the real questions: how much water, how good, and how reliable.
Construction records
Drillers file a well completion report with DWR. You can search DWR’s online map by address or coordinates, and if the report isn’t there, request it from the DWR region office for the county with DWR’s request form; there may be a fee. Napa County’s own files are searchable by parcel number or street address through the PBES public records search. The county notes that it is still digitizing records, so results may be incomplete; PBES Records can be reached at 707-253-4885 if something seems missing.
Yield
The county’s building permit standard is a useful benchmark. For a new dwelling or a bedroom addition on an existing well, PBES asks for a yield test report from the past five years, prepared by a licensed well driller, pump installer, geologist, engineer or Registered Environmental Health Specialist (REHS). If the yield is below 5 gallons per minute but at least 1, the county requires a domestic storage tank of 3,000 gallons per proposed living unit.
A buyer’s yield test during escrow won’t satisfy a future permit unless it meets those terms, so ask the tester to follow the county’s requirements if you plan to build.
Water quality
The State Water Board’s guidance for domestic well owners is direct: it is the owner’s responsibility to make sure the water is safe, and owners are encouraged to test regularly. Use a state-certified laboratory; Napa County publishes a partial list of California-approved labs, and the lab can recommend a test panel for the area and the well’s history. Ask the seller for any past results as well.
Shared wells
Many rural Napa homes share a well with neighbors. Once a well serves 5 to 14 service connections, it is a state small water system, which Napa County regulates; at 15 connections or 25 year-round residents it becomes a community water system. A winery can also become a public water system through its employees or visitors. For a well shared by two to four homes, the key document is a recorded agreement covering access, maintenance, cost sharing and what happens if the well fails. Our guide to private road maintenance agreements covers a similar kind of shared-cost arrangement.
New and replacement wells
Well permits are applied for through the county’s Online Permit Center. The well permit types listed by Environmental Health include Class IA or IB, Class II, monitoring wells, well reconstruction and well destruction only, along with a permit to connect water storage.
New wells are screened against the county’s groundwater standards, which look at distance from neighboring wells, springs and significant streams and, in some areas, at how much water the parcel already uses. In the Milliken-Sarco-Tulucay (MST) groundwater-deficient area east of the city of Napa, a project that uses groundwater also needs a groundwater permit from Environmental Health. Much of Coombsville lies in that area. The county has screened new wells under interim standards since January 2024, and a public draft released in March 2026 would tighten the valley floor limit, so the rules may change before you apply. The screening tiers and distances are explained in our groundwater rules guide; confirm with Environmental Health which standards apply before you budget for a well.
Septic systems: how Napa County permits them
Napa County’s septic program operates within the State Water Board’s statewide policy for onsite wastewater treatment systems (OWTS), first adopted in 2012 and most recently renewed on April 18, 2023. At the county level, the main steps are:
- Site evaluation. A qualified professional evaluates the soils, and Environmental Health staff inspect, to determine the size and type of system the site can support. If a system later has to go outside the evaluated area, a new site evaluation is required.
- Design and permit. The sewage permit application, electronic plans and plan check fee are filed online. Where an alternative system is needed, an engineer or REHS must design it, and the county must approve it before the building permit.
- Construction and inspection. The designer and the county inspect the installation.
Conventional, engineered and alternative systems
The county’s permit types include conventional systems, engineered layouts, alternative sewage treatment systems (ASTS), pretreatment units, and holding tanks or hold-and-haul. Its ASTS standards (revised July 2019, now listed as archival) describe pressure distribution, mound, at-grade, evapotranspiration-infiltration, subsurface drip, sand filter and aerobic treatment unit systems.
Under those standards, every ASTS needs at least three feet of acceptable soil above a limiting condition such as high groundwater, and new or modified ASTS enter the county’s monitoring program. Site plans must show a designated reserve area that stays protected and unencumbered, and a drip system needs room for a 200% reserve. For a buyer, the questions are whether the system is an ASTS, whether its monitoring is current, and where the reserve area is, because a reserve area limits where you can build a pool, barn or guest house.
Bedrooms, remodels and additions
Septic capacity is the hidden limit on many rural remodels. For a new dwelling or a bedroom addition, the county must approve the sewage disposal before it approves the building permit. The county code treats any room of at least 70 square feet that meets sleeping-room building codes as a “potential bedroom,” whatever it is called on the listing. Other remodels that add plumbing need a sewer line permit, a plot plan showing the septic system if one isn’t on file, and a compliant reserve area.
If you plan to add on or build a second home, read our guide to building an estate home in Napa County before you buy.
Is a well or septic inspection required to sell in Napa County?
In the Environmental Health pages we reviewed for this guide, we found no Napa County requirement for a septic or well inspection when a home is sold, so confirm with Environmental Health for the specific parcel. In practice, inspections are a matter of contract: the buyer’s inspection contingency is when you pump and inspect the tank, test the well’s yield and quality, and pull the county file. Our guide to well and septic inspections in Napa and Marin covers what those inspections include.
Sellers still have to disclose known problems. If you are selling a rural home, our guide to California seller disclosures explains what to include, and our overview of point-of-sale requirements shows which local rules apply in other jurisdictions.
Groundwater: the subbasin plan and the fee
The Napa Valley Subbasin, which underlies the valley floor, is managed under a groundwater sustainability plan carried out by the NCGSA. Most homeowners feel it through the fee. The NCGSA adopted the fee on December 9, 2025 (Resolution 2025-05) and, on August 11, 2026 (Resolution 2026-03), temporarily reduced all rates by about 55% for 2026-27, using $750,000 from the GSA’s fund balance and $500,000 from the County General Fund.
| User class | FY 2026-27 rate | Maximum authorized |
|---|---|---|
| Self-supplied user (mainly domestic wells) | $28.17 per parcel | $62.58 per parcel |
| Agricultural base rate | $17.53 per planted acre | $38.58 |
| Agricultural additional rate | $26.91 per groundwater-irrigated acre | $60.16 |
| Public water system | $58.47 per acre-foot | $129.87 |
- Who pays. Only groundwater users on parcels inside the subbasin. Agricultural and self-supplied users see it on the annual property tax bill.
- Who doesn’t. Parcels outside the subbasin, properties in the Calistoga, St. Helena, Napa and Yountville municipal water service areas, and properties that receive a municipal water bill are excluded, and customers of a water district or mutual water company don’t pay directly, though the provider may pass costs through.
- Waivers. Households at or below 80% of county median income could apply for a waiver, renewed each year, and dry-farmed acres pay only the base rate. The 2026-27 application deadline was July 10, 2026, before the rate cut, so ask the GSA (707-253-4471) about current options.
The reduced rate is temporary and applies to 2026-27; the county’s FAQs still show the maximum rates, so check the current figure on the tax bill when you budget. Parcels outside the subbasin boundary are outside the fee but still need county well permits, so confirm with the NCGSA whether a parcel is inside it.
A buyer’s checklist for a well-and-septic property
- Pull the county file through the PBES records search: septic permits, site evaluations, well permits and any notices.
- Find the well completion report on DWR’s map, or request it.
- Test yield and quality with a licensed professional and a certified lab, following the county’s yield-test terms if you plan to build.
- Pump and inspect the septic tank, and identify the system type, the leach field and the reserve area.
- Count potential bedrooms against the septic permit, not the listing.
- Read any shared-well agreement, and check whether the system has five or more connections.
- Check the tax bill for the groundwater fee and confirm whether the parcel is inside the subbasin.
- Map your plans (ADU, pool, barn, vineyard) against the leach field, reserve area and well setbacks.
Land buyers should also read our guide to Napa County land for sale, and anyone adding vines or a winery should work through our vineyard due diligence checklist.
This is general information, not legal or technical advice; talk to a licensed well driller or pump installer, a septic designer (engineer or REHS) or a hydrogeologist about a specific parcel.
Questions people ask
Do I need a permit to drill a well in Napa County?
Yes. New wells, well reconstruction and well destruction all need a permit from Napa County Environmental Health, applied for through the Online Permit Center. New wells are screened against the county’s groundwater standards.
How much water does a Napa County well need for a building permit?
For a new dwelling or bedroom addition, the county asks for a yield test from the past five years. A well producing less than 5 gallons per minute but at least 1 needs 3,000 gallons of storage per proposed living unit.
Does Napa County require a septic inspection when a house is sold?
We found no such requirement in the county’s Environmental Health materials, so confirm for the specific parcel. Buyers usually arrange a septic pumping and inspection during their contingency period.
Can I add a bedroom to a house on septic?
Only after Environmental Health approves the sewage disposal. Any room of 70 square feet or more that meets sleeping-room codes counts as a potential bedroom.
What is an alternative septic system?
Any system other than a conventional tank and leach field, such as a mound, at-grade, drip or aerobic treatment unit, used where soils or groundwater limit a conventional design. In Napa County, new and modified alternative systems enter the county’s monitoring program.
Do I pay the Napa groundwater fee if I have a well?
If the parcel is inside the Napa Valley Subbasin and you pump your own groundwater, yes: $28.17 per parcel for 2026-27, on the property tax bill. Parcels outside the subbasin, or served by a city water system, are excluded.
How do I find out when a well was drilled?
Search DWR’s well completion report map by address, or request the report from DWR’s region office. The county’s PBES records search may also have the well permit.
Who regulates a well shared by several houses?
A well serving 5 to 14 connections is a state small water system regulated by Napa County. The county’s water system page doesn’t describe rules for wells shared by fewer homes, so for those a recorded sharing agreement matters even more.
Well and septic questions are easier to answer before you’re in contract. Our team can help you pull the county file, line up inspections and read the results against your plans for the property; see how we work with Napa County buyers, or contact us about a specific parcel. Diamond may refer you to partners we work with, such as well and septic inspectors; you’re free to choose any provider.
Sources & notes
- Napa County Environmental Health: Well and onsite wastewater treatment
- Napa County Environmental Health: Building permit requirements for well and onsite wastewater systems
- Napa County: Design, construction and installation of alternative sewage treatment systems (revised July 2019, archival)
- Napa County PBES: Public records search
- Napa County Environmental Health: Public water systems
- Napa County: Environmental Health Division
- Napa County Groundwater Sustainability Agency: Groundwater sustainability fee
- California Department of Water Resources: Well completion reports
- State Water Resources Control Board (GAMA): Information for well owners
- State Water Resources Control Board: Onsite wastewater treatment systems (OWTS) policy
Last reviewed October 2026 by Matthew Martinez.
