Can my ADU share the main house's private well?

Usually yes, if the well produces enough water, the water is safe to drink, and your county environmental health department accepts the testing. Counties commonly want a well yield (pump) test and a water quality test before they clear a new dwelling for occupancy. The details vary a lot by county, so start with your environmental health office.

What counties typically check

• Yield. Nevada County, for example, uses a four-hour pump test and looks for at least 3 gallons per minute of sustained production. It may approve as low as 1 gallon per minute if a holding tank of at least 1,000 gallons is installed. Expect your county to consider the combined demand of the house and the ADU.
• Water quality. Nevada County requires a sample free of total and fecal coliform bacteria, analyzed by a state-accredited lab, plus a signed wellhead inspection report, before final occupancy clearance. Some counties test for more: San Bernardino County's shared-well guidelines also call for analysis of inorganic chemicals, total alpha activity, and general mineral and physical constituents.
• Setbacks. Wells must stay well away from septic components. El Dorado County requires 100 feet between a domestic well and the septic tank or leach lines.

Why it usually stays a private well

State drinking water rules for water systems start at higher connection counts. A system with 5 or more service connections is classified as a state small water system, and a public water system has 15 or more service connections or regularly serves at least 25 people at least 60 days a year. A house and one ADU on a single well fall well below those thresholds, so local rules usually govern.

If the ADU could ever be on a separate parcel

Sharing a well across parcels is a different case. San Bernardino County, for example, limits a shared well to four connections, requires a recorded water-sharing agreement and a recorded 10-foot easement, and calls for at least 3 gallons per minute of sustained yield per structure served.

Pressure and storage

A well system runs on a pump and pressure tank. Adding an ADU adds simultaneous demand and often a long service line, so you may need a larger pressure tank, a storage tank with a booster pump, or a bigger well pump. Booster pumps serving a water distribution system need a low-pressure cutoff under the plumbing code (CPC 609.9).

What it means for your plumbing plan

Your plans should show the well, pressure and storage tanks, the service line route and size to the ADU, setbacks from septic components, and any backflow protection. Size the ADU supply from fixture units and the actual pressure the well system delivers. See what utilities an ADU needs, fixture unit counts, and our plumbing plans.

Sources: nevadacountyca.gov, sbcounty.gov, eldoradocounty.ca.gov, waterboards.ca.gov.