San Bernardino County sizes a home septic tank by the number of bedrooms, from 750 gallons for 1 to 2 bedrooms up to 1,500 gallons for 5 to 6 bedrooms. Adding bedrooms or a second dwelling can change what the county requires.
The county's Information Bulletin IB-00018 and its septic FAQ both use the California Plumbing Code table for single-family homes:
The county's FAQ says that after the living room, dining room, family room, kitchen, bathrooms and utility rooms are set, any other habitable room of at least 70 square feet is counted as a bedroom. That includes dens, libraries, studies, game rooms, workshops and lofts. A room that can only be reached through another bedroom counts as part of that bedroom. A room open to a living area through a wide doorless opening of at least six feet is not counted.
The county reviews unclear cases one at a time.
The county FAQ says that if a system was oversized for future additions and a qualified contractor can certify it, no change may be needed. If only the minimum was installed, the tank must be replaced with the right size and the dispersal area increased. A lot may not be developed beyond what the soil can treat, and the 100 percent replacement area must still be met.
IB-00018 says a structure that adds wastewater flow to an existing septic system is treated as new development and re-evaluated by EHS. New developments need at least one-half acre per dwelling unit. On a lot of more than one acre with an existing home, a shared system is sized using a different column of the plumbing code table. On a half-acre lot, an advanced treatment unit may be required, and the added structure cannot connect to the main septic system without one. A junior accessory dwelling unit built inside the existing home is exempt from the table.
EPA lists tank size among the factors that decide how often to pump, along with household size, total wastewater and solids. A larger tank does not remove the need to pump. EPA says typical household tanks are pumped every three to five years.
Call a licensed septic contractor or designer before you add a bedroom, a guest house or a mobile home, so the system is sized before you build.
Check the permit records, the plot plan, or a past pumping or certification report. A septic professional can also locate the tank.
The county FAQ calls it a design rate set by a percolation test. It decides how much absorption area the system needs.
The 2018 county FAQ says yes if the original system was sized for it, the disposal area is increased by 30 percent and the lot meets the one-acre minimum. Confirm with the county.
The sizes come from the California Plumbing Code table. Inside a city, the city handles review, so check with its building department.
High Desert Septic Pumping is a free referral service, not a contractor. Request a free quote from a local septic contractor.