ADU Septic Systems: The Complete Guide

Key Takeaways

Some areas in San Diego County have limited sewer access. If you live in such an area, upgrading an existing system or adding a new system will likely be necessary to support your ADU. In some cases, you can tie into the existing system without any upgrades or modifications, but this depends on the current septic tank capacity, local building codes, and the condition of the current system.

If you do need to add a new system or replace a current system, this can cost as little as $4,000 or as much as $25,000, depending on the size and style of system you’re installing and the property’s needs.

What Is an ADU Septic System?

A septic system allows wastewater from toilets, showers, washing machines, and sinks to be removed from the home without the need for a traditional sewer system. While homes connected to a municipal system flow wastewater into the city sewer and, eventually, a treatment plant, septic systems store wastewater in a big container under the ground and then slowly distribute that wastewater back into the ground over time through what’s called a drain field.

Proper system sizing starts with understanding your ADU’s wastewater needs. Always consult a pro before excavation to avoid design missteps.

Why Consider a Septic System for Your ADU?

Typically, septic is not a choice but a necessity based on your location. Some municipalities have city water and sewer systems; others don’t. Property owners living in rural areas that don’t have a city sewer system have to install septic systems for primary residences and accessory dwelling units.

However, if you do live in an area that relies on septic, there are some benefits. Septic systems are considered more sustainable than traditional sewer. More than that, they can save you on utility costs. Though septic systems do involve annual maintenance, these fees are usually much less than typical sewer and water usage bills.

Septic systems offer self-sufficiency—ideal for remote or sewer-limited lots. A well-installed system supports long-term performance and value.

Can You Tie Into an Existing Septic Tank for Your ADU?

Yes! If you already have a septic tank, you can tie the plumbing from your new guest house, in-law suite, or rental property into your existing septic, so long as it meets a few requirements.

• Does your current system have the capacity? Septic tank capacity is usually determined by the property’s total bedroom count. So if you have a 3-bedroom septic system for your 3br house and your ADU has 2 additional bedrooms, your current system doesn’t have the capacity to support your ADU. If you aren’t sure what the capacity of your tank is, get the opinion of a licensed contractor or septic consultant.
• Is your current system in good shape? Verify the existing tank and leach field are in good condition and free of issues like pooling or odors. Also confirm that your current system meets any local building codes.
• Will your building department allow you to tie in? Even if your tank can support an ADU, you need to confirm that local building codes and septic requirements allow you to tie into an existing system, either with a dedicated connection or distribution box.

If your current system doesn’t meet these requirements, you’ll need to talk about how you can upgrade or augment your current system to support the additional waste created by your ADU. This typically involves:

• Replacing or enlarging the system, especially if the septic size for the existing tank is too small
• Adding a separate septic tank that’s dedicated to the ADU

These solutions often involve a septic permit and/or a septic contractor.

Tying into an existing septic system can save costs—but only if your current setup meets capacity, condition, and code requirements. Always get it inspected first.

Types of Septic Systems for ADUs

If you do need to replace or supplement your primary septic, there are several options available to you, with seepage pits/leach fields being the most popular, though other solutions, like aerobic treatment units, are more efficient, especially in poor soil conditions.

Conventional Septic Systems: Seepage Pit vs. Leach Field

With conventional septic systems, waste is collected in a septic tank, then naturally (and safely!) filtered back into the ground over time via a drain field.

Advanced Treatment Systems

If there are space constraints on your property that don’t allow for a larger conventional system or you have poor soil quality, you can opt for an advanced treatment system or ATS. These systems use oxygen to break down wastewater more efficiently.

Alternative Options

Typically, most clients we work with opt for a conventional system or an ATS, but there are other options available, such as chamber systems, drip distribution systems, and mound systems. These systems can help overcome problems like shallow soil depth, uneven terrain, elevation differences, or high groundwater. If you believe your ADU project may require an alternative septic solution, a septic contractor can advise you on the best option for your property.

System type matters. Conventional drain fields work for many, but advanced treatment units can solve soil and space challenges efficiently.

Regulatory Requirements for ADU Septic Systems in California

As with any other aspect of ADU construction, there are certain regulatory requirements to keep in mind when it comes to septic system installs. These include:

• Installing a septic system at least 5 feet from a structure, at least 100 feet from any wells, and at least 50 feet from sources of groundwater (though many municipalities can have even larger distance requirements)
• A setback requirement of at least 5 feet from the property line
• Minimum lot size of half an acre

What is the Minimum Lot Size for a Septic System in California?

In many cities in the San Diego area, the minimum lot size for a septic system is half an acre, but this can vary based on soil conditions and the availability of public sewer and water.

Typically, cities that have city water and sewer have minimum lot sizes of half an acre or less. But if you live in an area without public sewer or with poor soil conditions or high groundwater, the minimum lot size for septic can be as large as an acre. If you live near conservation land or other environmentally protected areas, the minimum lot size can also be larger.

Lot size isn’t just a number—it dictates septic feasibility. In many areas, half an acre is the bare minimum depending on soil and groundwater conditions.

How Much Does an ADU Septic System Cost?

The average cost of a septic system depends on several factors. Of course, the style of septic you’re installing plays a role, with certain systems being more expensive. But there are other factors to consider, too, like septic size, soil type, and overall terrain. Poor soil conditions or uneven terrain often result in additional costs.

These costs also don’t reflect other aspects of septic installation, like soil evaluations and septic permits, or ADU plumbing costs.

What Is the Timeline for Adding a Septic System to Your ADU?

When determining the timeline for adding a septic system to your property, there are 2 main factors to consider: permitting and actual system installation.

When it comes to permits, your plans need to be approved by the Department of Environmental Health. This typically takes between 60-90 days. If, for whatever reason, your plans are rejected, or modifications are requested, it can take another 60-90 days to approve after resubmission.

When it comes to the actual installation of the septic system, most projects take between 2-3 weeks to prep the site and get the system up and running. Once the work is complete, the site needs to be inspected. This can add another 1-2 weeks to your timeline, depending on inspector availability.

Plan early. Permitting can take up to 90 days, and install adds weeks. Build septic timelines into your ADU construction calendar to avoid delays.

Can You Decommission Your Septic System and Connect to a Sewer Line Instead?

Yes. If you live in an area with a municipal sewer system, you can decommission your standard septic and connect both your primary dwelling and your accessory dwelling unit to the sewer, so long doing so doesn’t go against any local regulations. Know that the costs for this are significant, as much as $40,000 or more! For many homeowners, the cost is worth it for the ease of being on city sewer, but if budget is a concern, it’s definitely something to take into consideration.

Maintenance Tips for ADU Septic Systems

Property owners with septic systems need to maintain their systems every year. Septic maintenance isn’t hard or overly expensive, but it’s important to prevent failures, extend the life of the system, and avoid costly repairs.

Routine Maintenance

Septic tanks generally need to be pumped every 3-5 years, though this varies depending on your exact system. They also need to be inspected every 1-3 years to check for leaks or other issues.

Avoiding Common Problems

Between routine maintenance events, homeowners also need to take care to prevent other common septic problems, like system overload or clogs.

Low-flow toilets, shower heads, and faucets are all essential in homes that run on septic to reduce water and avoid system overload. Also, nothing besides toilet paper (no baby wipes, paper towels, or even feminine hygiene products) should be flushed into the system.

On the outside of the house, avoid planting anything but grass above the drain field, as roots from trees or flowering shrubs can lead to blockages. Rain spouts and gutters should also be directed away from the drain field.

Septic system health = long-term peace of mind. Routine inspections and proper water use habits are your best defense against backups and system failure.

Unlock the Potential of Your ADU With Better Place Design & Build

Having to consider septic requirements on top of all the other aspects of ADU construction can feel overwhelming. Luckily, you don’t have to manage it alone. At Better Place, we seamlessly manage all construction details, including those having to deal with septic permitting and installation, to keep a project running on time and on budget. To get more details on the actual construction of your ADU, use our cost calculator or—better yet—schedule a free project consultation.

A well-planned septic system is just one piece of the puzzle. Our turnkey service ensures every piece—from design to landscape—fits together seamlessly.