Septic tank size by state / Pacific
California septic tank size
California has no statewide minimum septic tank size. The tank for a house is set by the county or local health department that issues the permit, so the answer depends on where the house is. This page explains who decides, quotes the state source, and shows what neighbouring states require for comparison.
What the California rule says
The rule is Water Quality Control Policy for Siting, Design, Operation and Maintenance of Onsite Wastewater Treatment Systems (OWTS Policy), Tier 1 §8.2 and Tier 2 (Local Agency Management Programs); eff. May 13, 2013, administered by State Water Resources Control Board (OWTS Policy), implemented by Regional Water Boards and county/local agencies. In its own words:
In order to address local conditions, Local Agency Management Programs may include standards that differ from the Tier 1 requirements for new and replacement OWTS contained in Sections 7 and 8.
Source: Water Quality Control Policy for Siting, Design, Operation and Maintenance of Onsite Wastewater Treatment Systems (OWTS Policy), Tier 1 §8.2 and Tier 2 (Local Agency Management Programs), read 2026-10-20.
- The State Water Board's OWTS Policy sets construction standards for tanks but no statewide volume.
- Each county or local agency sets tank size in its own Local Agency Management Program, so the answer depends on the county.
- The Policy assumes an average single-family home produces 250 gallons a day; that is a flow assumption, not a tank size.
How California compares with nearby states
Alaska requires 1,000 gallons for three bedrooms; Hawaii requires 1,000 gallons for three bedrooms; Oregon requires 1,000 gallons for three bedrooms; Washington requires 1,000 gallons for three bedrooms.
Before you size a tank in California
- Bedrooms, not people, set the size in most rules. Rooms that could be used as bedrooms are usually counted, so check how your permitting office counts a den or a finished basement.
- Garbage disposals, grinder or ejector pumps and large tubs change the answer in many states; the points above list California's adjustments.
- The tank is only half the system. The drainfield is sized separately from soil tests, and it is usually the bigger cost.
- A private well on the same lot has its own setbacks from the tank and field; the well pump and pressure tank calculators size the water side.
Figures are the state's minimum as written on 2026-10-20. Local health departments can require more; the permit issued for your property governs. See the methodology for how each state was read.