Well and Septic Inspections When Buying in Napa or Marin County

When you buy a rural home in Napa or Marin County, neither county requires a well or septic inspection at sale, so the buyer’s inspection contingency is where both systems get checked. A complete check has four parts: the county records, a septic inspection with the tank pumped, a well yield (flow) test and a water quality test from a certified laboratory. Each answers a different question, and each takes time to schedule.

The rules behind those tests differ by county. Napa County ties well yield to its building permit standard and charges a groundwater fee in the valley subbasin; Marin grades septic systems by class and requires Title 22 water testing for a domestic well permit; and Stinson Beach runs its own septic program. This post is the practical companion to our county guides to Napa County wells, septic and groundwater and Marin County septic systems, which cover the permit rules in depth.

Why the inspection period carries the weight

Marin County Environmental Health Services (EHS) says it answers questions about septic systems during a sale and publishes a residential inspection form for performance evaluations, but it does not require an inspection before a sale. We found no point-of-sale septic or well inspection rule on Napa County’s Environmental Health pages either. Stinson Beach adds one wrinkle: its water district reissues septic discharge permits when ownership changes, but that is a permit step, not an inspection.

The disclosure laws still apply. A seller has to disclose known problems with the well or septic system, but a disclosure tells you what the seller knows, not how the system is performing. That is why the tests below belong in every rural offer, and why they should be ordered in the first days of escrow.

Step 1: Pull the records before anyone digs

Records tell you what was permitted, which tells the inspector what to look for.

Ask the seller for pumping receipts, repair invoices, past inspection reports, any well yield and water test results, and, for an alternative system, the current operating permit and monitoring reports. Missing paperwork is itself a finding.

Step 2: The septic inspection

The U.S. Environmental Protection Agency says a typical household system should be inspected at least every three years and pumped every three to five years, and that systems with mechanical parts should generally be inspected once a year. During service, the provider inspects for leaks and examines the scum and sludge layers in the tank.

For a purchase, ask the inspector in writing what the scope includes. Questions worth asking:

• Will the tank be pumped so its walls, baffles and outlet can be seen, and will the inspector be there when it is?
• Will the inspector locate the leach field and the reserve area and check for wet spots or surfacing effluent?
• For a pump, mound, drip or aerobic system, will the inspector test the pump, alarms and controls?
• Will the report compare the system with the county permit, including bedroom count?
• In Marin, will the evaluation be written on the county’s residential inspection form?

Bedroom count is where many rural deals change. Napa County Code defines a “potential bedroom” as any room of 70 square feet or more, including lofts, offices and game rooms, and says a closet doesn’t decide it. If the listing shows more bedrooms than the septic permit supports, assume that gap will matter to the next permit you apply for.

Stinson Beach: district permits come first

In Stinson Beach, the Stinson Beach County Water District regulates onsite systems under Title IV of its code. Pumping a tank counts as maintenance and needs a district maintenance permit, valid for 30 calendar days, and pumping contractors need both a district pumping permit and a county license. Discharge permits are recorded with the County Recorder and reissued when ownership changes. Build that lead time into the inspection schedule, and ask the district about the parcel’s permit status. Our Stinson Beach guide covers the town.

Step 3: The well yield test

A yield test measures how much water the well can deliver. For a buyer, it answers whether the house will have enough water in late summer, and whether it could support more later.

Napa County’s building permit standard is the most useful benchmark here. When an existing well will serve a new dwelling or an added bedroom, the county wants a yield test no more than five years old. A licensed well driller, pump installer, geologist or engineer, or a Registered Environmental Health Specialist (REHS), must prepare it. A result below 5 gallons per minute but at least 1 gallon per minute requires 3,000 gallons of domestic storage per proposed living unit.

Two practical points follow:

1. Have the test done to permit standard. If you might add a bedroom, a guest house or an ADU, ask the tester to follow the county’s terms, so the report can serve a future permit within five years.
2. Read the season. A test in a wet spring and a test in September can tell different stories. Ask the seller for older results and when they were taken.

Marin’s EHS pages we reviewed don’t publish a yield standard for existing wells, so ask the department what it would require for the project you have in mind.

Step 4: The water quality test

The State Water Resources Control Board says it is the well owner’s responsibility to make sure domestic well water is safe, and its groundwater program notes that the state doesn’t regulate domestic well water quality. In other words, nobody checks a private well for you.

What to test:

• The EPA baseline. EPA recommends testing a private well every year for total coliform bacteria, nitrates, total dissolved solids and pH, and testing right away after flooding, nearby construction, a well repair or a change in taste, odor or color.
• Local contaminants. EPA says the local health department can tell you which contaminants are common in your area. The State Water Board’s domestic well project has sampled for coliform bacteria, general minerals, metals including lead and arsenic, nitrate and organic compounds.
• Marin permit standard. To permit a well as a domestic water supply, Marin County requires the water to be tested against the state’s Title 22 drinking water standards, and results over the maximum levels need a written treatment plan before the permit is issued.

Use a laboratory certified for drinking water testing, as EPA advises, and have the sample drawn by the lab or a professional who follows its chain-of-custody rules. If the house has a treatment system, ask for a sample before and after treatment so you know what the equipment is doing.

Diamond may refer you to partners we work with, such as septic inspectors, well testers and laboratories; you’re free to choose any provider.

Napa groundwater: the ongoing cost

In Napa County, a buyer also inherits a groundwater bill. Parcels inside the Napa Valley Subbasin that pump their own groundwater pay a sustainability fee on the property tax bill. For fiscal year 2026-27 the self-supplied rate is $28.17 per parcel, after the agency cut rates by about 55% on August 11, 2026; the maximum authorized rate is $62.58. Parcels outside the subbasin and those served by a city, district or mutual water company don’t pay it directly.

Inside the Milliken-Sarco-Tulucay (MST) groundwater-deficient basin, a project that uses groundwater also needs a county groundwater permit before a building permit. If you plan to add vines or a second home, read our guide to Napa County groundwater rules before you remove contingencies.

How results limit what you can build

Well and septic findings decide more than whether the house works today. They often decide what you can add.

If a guest house is the reason you’re buying, read our post on homes with guest houses in Napa, Sonoma and Marin first.

A timeline that fits a typical contingency period

1. Days 1 to 2: request county records and the seller’s maintenance file; book the septic inspector, the well tester and the lab.
2. Days 3 to 7: septic pumping and inspection; well yield test. In Stinson Beach, get the district maintenance permit before pumping.
3. Days 5 to 10: water sample drawn; lab turnaround varies by test.
4. Before contingency removal: compare results with your plans, get bids for any repair or treatment, and negotiate a repair, credit or price change.

If the schedule is tight, ask for a longer investigation period in the offer. For the same checks in Sonoma County, see our guide to wells, septic and rural utilities in Sonoma County; for rural towns, see Pope Valley, Angwin and Inverness, and our Napa County and Marin County real estate guides.

This is general information, not legal or engineering advice; for a specific property, rely on a licensed well driller or pump installer, a qualified septic inspector or designer, and a certified laboratory, and talk to a California real estate attorney about contract terms.

Questions people ask

Is a septic inspection required to sell a house in Marin or Napa County?

Not by either county for a typical sale. Marin’s Environmental Health office says it doesn’t require one, and we found no such rule on Napa County’s pages. Buyers usually make inspections part of their contingency, and sellers must disclose known problems.

What does a well inspection for a home purchase include?

Usually a yield (flow) test, a look at the pump, pressure tank and wellhead, and a water quality test from a certified lab. Add the county well permit and the state well completion report to the file.

What should I test well water for?

At a minimum, total coliform bacteria, nitrates, total dissolved solids and pH, which EPA recommends testing every year. Ask the county health department about local contaminants such as arsenic.

How much water does a well need to produce?

For a Napa County building permit, a well producing less than 5 gallons per minute but at least 1 needs 3,000 gallons of storage per living unit. Marin’s pages don’t publish a yield figure for existing wells.

Do I need a permit to pump a septic tank in Stinson Beach?

Yes. The Stinson Beach County Water District treats pumping as maintenance requiring a district permit valid for 30 days, and pumpers need a district permit and a county license.

Does a Napa well owner pay a groundwater fee?

Inside the Napa Valley Subbasin, yes: $28.17 per parcel for 2026-27 on the property tax bill. Parcels outside the subbasin and those on public or mutual water don’t pay it directly.

If you’re buying a rural property in Napa or Marin, we can line up records requests and inspections from the day your offer is accepted, and help you weigh the results against your plans. See how we work with buyers.

Sources & notes

• County of Marin Environmental Health Services: Septic systems (updated October 8, 2026)
• County of Marin Environmental Health Services: Private wells
• County of Marin Environmental Health Services: Domestic water supply
• Napa County Environmental Health: Building permit requirements for well and onsite wastewater systems
• Napa County Groundwater Sustainability Agency: Groundwater sustainability fee
• Stinson Beach County Water District: Title IV Onsite Wastewater Management Code
• State Water Resources Control Board (GAMA): Information for well owners
• State Water Resources Control Board (GAMA): Domestic Well Project
• U.S. EPA: Protect your home’s water (private wells)
• U.S. EPA: How to care for your septic system

No inspection or testing prices are given; they vary by provider, system type and access, so get written quotes.

Last reviewed October 2026 by Matthew Martinez.