California made ADUs legal almost everywhere — but if your property runs on septic, the county will not issue a building permit until you prove the septic system can handle the extra bedrooms. That proof step is where ADU projects stall, and it is exactly the work we have done since 1950.
Here is the core rule: septic capacity in California is sized by total bedroom count on the parcel, not by the number of buildings. A 3-bedroom house with a 2-bedroom ADU is treated as a 5-bedroom load. If your existing tank and leach system were sized for 3 bedrooms, the county will require an upgrade before the ADU is approved.
How the ADU Septic Process Works
- System evaluation. We locate and assess your existing tank and leach system and pull whatever records the county has.
- Capacity determination. Total future bedrooms vs. rated capacity. Sometimes the existing system passes as-is — that is the cheapest possible outcome, and we will tell you honestly if you have it.
- Soil testing if needed. Expanding leach capacity usually requires current PERC or soil-profile data. We test to your county’s protocol.
- System design. We design the upgrade — upsized tank, expanded leach lines, seepage pit, or ATU where soil demands it — as permit-ready plans.
- Permits and construction. We submit to the county, build with our own equipment, and meet the inspector.
County by County
San Bernardino County reviews ADU septic through Environmental Health Services and generally follows bedroom-based sizing with required setbacks from the new structure. Riverside County DEH runs one of the stricter programs and commonly requires soil data for any dispersal expansion. Los Angeles County layers regional water-board rules on top for many parcels. Orange County ADUs on septic — mostly canyon and estate properties — go through county health review. San Diego County ADUs on septic go through the county’s Department of Environmental Health and Quality. We work in all five counties and keep current on each one’s quirks.
The Mistake That Costs ADU Builders the Most
Designing the ADU first and checking septic last. If the septic evaluation happens after architectural plans are drawn, a required leach field expansion can collide with where the ADU was placed — setback rules mean the dispersal area and the structure fight for the same ground. Bring us in before the site plan is final and we will tell you where the septic needs to live, so you only draw the plans once.
Get an ADU Septic Evaluation →Related: PERC & soil testing · Inspections & certifications · Septic FAQ
ADU Septic Questions
Can I build an ADU on a property with a septic system in California?
Yes. California law allows ADUs on septic properties, but your county will require proof that the septic system can handle the added wastewater before issuing the building permit. That usually means an evaluation of the existing system and, depending on bedroom count and soil, an upgrade to the tank, the leach system, or both.
Does adding an ADU require a bigger septic tank?
Often, yes. Septic sizing in California is driven by total bedroom count across all dwellings on the parcel. If your main house and ADU together push past your tank's rated capacity, the county will require an upsized tank and possibly expanded leach capacity. A one-bedroom ADU on a property with a generously sized existing system can sometimes pass without changes — the evaluation determines it.
Do I need a PERC test for an ADU?
If the leach system must expand, most counties require current percolation or soil-profile data to size the new dispersal area. If the existing system has documented capacity for the added bedrooms, testing may not be needed. Counties differ — San Bernardino, Riverside, Los Angeles, Orange, and San Diego each run their own review — and we handle the requirements for all five.