By Jennifer Bardenheuer, REHS
If you live on a septic system and you're thinking about adding a bedroom, building an ADU, or putting a bathroom in the shop, the septic system is one of the first things to look at. It's often the last thing people think about. A remodel can be designed, priced and drawn up before anyone asks whether the system in the ground can handle it. When the answer is no, plans get redrawn, budgets change, and projects stall.
Here's what I look at when a client asks me, "Can my septic handle this?"
Septic capacity is about bedrooms, not just people
A septic system is sized for a "design flow," the amount of wastewater the house is expected to produce on a busy day. For a home, that design flow is generally based on the number of bedrooms, because a bedroom is a reasonable stand-in for how many people could live there over the life of the house. Two people in a four-bedroom house today could be a family of six tomorrow.
So, adding a bedroom, even one you plan to use as an office, may raise the design flow the county expects your system to handle. An ADU, which is a separate living unit, adds flow of its own. A bathroom in a shop or garage may also count, depending on the rules of your local jurisdiction.
The first questions to answer
Before I can tell you whether an addition will work, let's figure out what you already have. These are the questions I start with:
How old is the system, and what shape is it in? An older system that's working fine for today's use may not have much lifespan to take on more.
Is there an original permit or an "as-built"? An as-built is a drawing of what was actually installed. Counties often have these on file, and they can save a lot of guesswork.
How big is the septic tank? Tank size is tied to design flow, so a tank that fits a three-bedroom house may be too small for five.
How big is the leach field, and what type is it? The leach field (also called a disposal field or drain field) is where treated wastewater soaks back into the soil. Its size and design matter as much as the tank.
Is there a reserve or expansion area? Many systems were permitted with an area set aside for a replacement or added leach field. An addition to the existing system also means that we have to account for future replacement area for the addition. Many counties will want to make sure there is room for both.
What about setbacks? The system has to stay a required distance from wells, creeks, property lines, buildings and slopes. A new addition, driveway or ADU can't be built on top of the leach field or too close to it.
What are the soil and groundwater like? Soil conditions decide how much area a leach field needs. If they've never been documented, or the original records are thin, a soil profile or percolation (perc) test may be needed.
How the county looks at an addition
Every county handles this a little differently. Some want a full evaluation of the existing system before they'll sign off on a building permit. Others look mainly at whether the bedroom count is going up. Some have specific rules for ADUs. Because the requirements vary from one jurisdiction to the next, I don't assume. I look at what your county requires and walk you through that process.
In general, though, expect the county's environmental health department to review the septic side of any project that adds bedrooms, a living unit or plumbing fixtures. It helps to know what they'll ask for before you submit.
Keep it, fix it, or replace it?
Once we know what's in the ground and what the county requires, the answer usually falls into one of three groups.
The existing system can stay as it is. If the tank and leach field were sized for more bedrooms than you have now, or the addition doesn't raise the design flow, you may not need to change anything. The county may still want proof the system is working.
The system needs repair or expansion. You might need a larger tank, a second tank, more leach field or repairs to failing parts.
A new system is the better answer. If the system is older, failing, undersized, or there's no good room to expand it, putting money into it may not make sense. Sometimes a new system, designed for the house you'll actually have, is the cleaner and longer-lasting choice. For an ADU, a separate system may be an option in some cases.
A few tips before you start
Check the septic before you design. Have the system evaluated before you pay for house plans. Where the leach field and reserve area sit can shape where an addition or ADU can go.
Check before you buy. If you're buying a property with plans to add on, find out what the septic can handle before you close.
Pull the records. Ask the county for any septic permits or as-builts for your parcel. If you can't find them, I can help track them down.
Don't build over it. Keep driveways, foundations and heavy equipment off the tank, leach field and reserve area.
Let's figure it out before you build
Adding on is often very doable. Let me help you with a smooth permitting process by answering all of the septic questions first. I help homeowners and buyers across California, including Tuolumne, Calaveras, Mariposa, Yolo, Amador, Mono, Contra Costa and Alameda counties, figure out what their system can handle and what the county will need.
If you're thinking about adding on, give me a call at (209) 536-8048. We can talk through your project and the best next step.
— Jennifer Bardenheuer, REHS
