Determining whether or not your accessory dwelling unit (ADU) needs a septic system is one of the least glamorous parts of the design and build process for your guest house. While many people have a general idea of what a septic system is, the details of what exactly is needed to accommodate an ADU can be unclear. So, let’s talk septic!
What is a Septic System?
A septic system is simply a way to distribute waste into the earth. Septic systems are more technically known as underground waste treatment systems. They get rid of all the stuff you don’t want to hang around – or underneath – your new ADU. Septic systems are most common in more rural areas, where sewer lines are more difficult to get to.
Every style of septic system will have a septic tank and a drainfield. A septic tank is exactly what it sounds like: a large container that stores all waste inside, until everything is pumped out and disposed of. You have probably dealt with a septic tank before if you have ever owned or used an RV. It is imperative your tank is pumped regularly. This is because any solid material will build up over time, despite having a drainfield. Liquid waste will pass through the tank and move into the second piece of your system – the drainfield.
Do Your ADU Need a Septic System?
No! Septic systems are only necessary if your property is not connected to a city sewer system (read more about ADUs with sewer). If this is the case, all waste-disposal responsibility falls on you, the property owner, and your septic system. This is most common in more rural areas where a central sewer system is unrealistic, difficult to connect to, or some mix of both. If you live in one of these areas, it is likely you are already familiar with septic systems and may even have one of your own already. This then begs the question:
Can I Use My Existing Septic System for an ADU?
Many people ask if you can add an ADU to the existing septic system that serves the primary residence, and it depends on several factors. The most likely answer would be “no”. This is because main-house septic systems are rarely large enough to accommodate both a primary dwelling and an ADU. However, if you feel confident in your existing septic system, the best course of action would be to discuss this possibility with your septic contractor and examine what your options are. In some cases, increasing the capacity of the existing septic system may be an option. A feasibility report from a septic provider will produce the needed information.
How much space do I need for a new septic system?
The installation of a new septic system comes with specific spatial prerequisites to ensure proper function and compliance with health regulations. Here’s what you need to consider:
- Minimum Lot Size: Generally, if your property is less than half an acre, it may be too restrictive for a standard septic system due to the required setbacks from property lines, wells, and water bodies.
- Drainfield and Reserve Area: A typical system, including reserves, requires 5,000-10,000 square feet of usable land. Properties with marginal soils may require 20,000+ square feet, especially for larger ADUs with multiple bedrooms. This area should typically not be currently utilized for other purposes, such as driveways or patios, as the septic system requires dedicated space for components like the leach field.
- Setbacks and Buffer Zones: In addition to the space for the septic tank and leach field, it’s important to consider the required setbacks from other site features. Septic systems must be placed 10 feet from structures, 100 feet from wells or water bodies, and additional distances depending on dispersal methods.
It’s important to note that these are general guidelines, and local regulations may impose more stringent requirements. Consulting with a septic system professional and conducting a site evaluation are critical steps in determining the specific space needs for your property.
What Are the Key Septic Changes In Effect in 2025?
As of February 1, 2025, changes from the new Local Agency Management Program (LAMP) for Onsite Wastewater Treatment Systems (OWTS) will go into effect. Here are the most critical updates:
Flow Capacity and Bedroom Limits
The updated regulations impose strict limits on the daily wastewater flow that OWTS can handle:
- Standard OWTS: Maximum of 900 gallons per day (GPD), equivalent to 6 bedrooms total across the primary residence and ADU.
- OWTS with Supplemental Treatment: Maximum of 1,500 GPD, supporting up to 10 bedrooms total.
This means that if your existing home already has a large number of bedrooms, your ADU size could be limited unless you install a supplemental treatment system.
Annual Operating Permit Requirement
Homeowners using OWTS must now renew an annual operating permit. This includes maintaining operation and maintenance records and ensuring compliance with ongoing requirements. Regular pumping and retention of pumping records for six years are now mandatory. Enhanced maintenance requirements, including an operations and maintenance plan for systems with supplemental treatment, are also in place
These changes are designed to protect water quality but add substantial complexity and cost to septic system projects. The full set of rules and all details can be found on the County of San Diego’s Environmental Health & Quality Septic Systems page. We are only just beginning to understand the changes and how they will play out in the real world. Whether you’re upgrading an existing system, installing a new one, or considering sewer connection, early planning and professional guidance are critical.
Can I abandon my septic system and tie into sewer?
Yes, you can abandon your septic system and tie into the municipal sewer system, provided that the public sewer is accessible from your property and you comply with local regulations and requirements. We’ve had many clients elect to connect both their primary dwelling unit and their new accessory dwelling unit to sewer, and we coordinate that entire process.
It’s important to note that the costs for sewer connection can be significant, including permit fees, contractor costs, and any sewer system development charges imposed by the municipality. Typically we see these total costs in the range of $40K. However, connecting to a public sewer can increase property value and eliminate the maintenance and potential environmental issues associated with septic systems.
Steps to Tie into Sewer
- Assessment and Permission: Contact your local sewer authority or public works department to assess the feasibility of connecting to the public sewer. They will provide information on the process, costs, and any specific requirements or restrictions.
- Permits: If sewer connection is feasible, permits from the local municipality will need to be obtained. This will involve submitting detailed plans and paying associated fees.
- Abandonment of Septic System: Properly abandoning your septic system is crucial to prevent environmental contamination. This usually involves pumping out all remaining waste, filling the tank with an inert material like sand or gravel, and possibly removing or collapsing the tank. Local health department guidelines will dictate the exact process.
- Connection to Sewer Line: A licensed contractor will need to install a new sewer line from your home to the main sewer, ensuring it meets all code requirements for depth, materials, and slope.
- Inspection and Approval: Once the work is completed, it will need to be inspected by local authorities to ensure it complies with all regulations. After passing inspection, the dwellings may officially use the sewer system.
- Disconnection and Restoration: After successfully connecting to the sewer, any remaining parts of your septic system will need to be properly disconnected and sealed off . The area can then be restored, which may involve landscaping or other property repairs.
How Long Does It Take to Add a Septic System?
Permitting: Septic plans must be approved by the local Department of Environmental Health, a process that typically takes 60-90 business days.
Installation: Once approved, installation takes about 2-3 weeks. The system is then tied into your ADU’s plumbing, which completes the final step.
Read about the overall timeline to add an ADU.
Engaging a Septic Contractor: The Path to Clarity
To explore the potential of your septic system supporting an ADU, a specialized septic contractor should be an initial point of contact. Their expertise will be invaluable in assessing the feasibility and outlining the necessary steps.
Primary Considerations
- Bedroom Count: The size of a septic system is typically governed by the number of bedrooms rather than bathrooms. This is because bedrooms imply potential occupancy, which in turn indicates the volume of wastewater. Inform your contractor about the number of bedrooms you plan to include in your ADU floor plan.
- System Mapping and Condition Assessment: Your contractor will need to conduct a thorough mapping of your existing septic system. This includes locating all components and evaluating their current condition to ensure they can handle additional usage.
- System Type and Placement Proposals for the ADU: At this stage, a full design isn’t necessary. Instead, the contractor will suggest appropriate system types and potential locations for the ADU’s septic needs. This preliminary proposal will help in understanding the scope of work and whether it’s feasible within your property constraints.
Evaluation
The ground in your property will be tested to determine its percolation, or the rate at which liquid can move through the soil and make its way into the groundwater, during the introductory percolation consultation. This is an important step to ensure waste does not build up on your property. If the percolation is deemed subpar or poor, the entire build may be deemed infeasible. Luckily, all testing will be done well before any actual construction begins on your property, along with the rest of the necessary feasibility studies, to ensure minimal money and time is wasted.
With this septic feasibility information in hand, you can continue the broader discussions about your ADU design & scope of work. Your general contractor should work closely to coordinate planning for both the septic system and the ADU build. Contact us so we can help you get in touch with providers and coordinate your ADU feasibility with this work. This first step will direct you towards the system that works best with your budget and property and makes construction as seamless as possible.
You can also read more about onsite wastewater treatment system (septic system) permitting process and design criteria for the County of San Diego.
How Much Does a Septic System Cost?
The system you choose will be based on a number of different factors like the location of your property, amount of space available, and the quality of the soil in your area. The cost of septic tank installation varies by city and state. San Diego and nearby residents can expect the process and installation to be more expensive than other parts of California.
A good septic contractor will explain considerations or requirements that might arise with different system options. There are many different types of drainfields, described below:
Leach Field
This system is often referred to as the “traditional septic system” because it is the most common. A leach field is the most eco-friendly method of waste distribution, because it allows for the recycling and replenishing of liquids back into the ecosystem through the use of perforated pipes. This style of septic is generally the most cost-effective as well. They average around $25-30K for design and installation for a system to accommodate a typical ADU. However, leach fields also take up the most underground space, so it is not always a viable option. Certain lots present challenges for a traditional leach field, so there are alternative septic systems that can be utilized.
Presby System
Most ADU septic projects will require a Presby system due to constrained space/site conditions. However, in some cases, a conventional system or expansion of the existing system may be installed to accommodate an ADU. Similar to traditional systems, Presby systems need an initial inspection and certification, which costs $2,000–$3,000. However, Presby systems also require recurring inspections and permit fees every six months for as long as they’re in use. The Department of Environmental Health requires property owners to sign a legally binding contract with a certified Septic Engineer to test Presby systems. If the property is sold or transferred, the new owner must sign a new contract.
Vertical Seepage Pit
A vertical seepage pit acts in a similar manner to a leach field. The difference is a large concrete cylinder takes the place of a whole field of pipes. This system is typically used in situations where the space available on the property is not large enough for a leach field. The pit is often buried about six feet under the soil and extends deep into the ground, where bacteria actually dispose of all waste. This style of septic is more heavily regulated than leach fields, because your waste is not processed to the same extent as with a leach field. Vertical seepage pits are often allowed only in coastal settings where groundwater is mixed and purified with saltwater. See a map of where vertical seepage pits are allowed in San Diego County.
Horizontal Seepage Pit
Horizontal pits are very similar to vertical pits. However, it is worth noting that horizontal seepage pits require deep soil and good percolation, which we will discuss later. Both horizontal and vertical seepage pits are notably more expensive than leach fields. Horizontal pits cost around $60-70K and are only used in locations of limited space.
When can I get a septic permit in San Diego County? Do I need a building permit?
In San Diego County, to obtain a septic permit, you must first have an approved set of building plans. These plans should accurately reflect the number of bedrooms indicated on the septic layout approval. This ensures that the septic system is designed to accommodate the specific needs of your property. Read more about choosing the right size ADU.
Navigating the complexities of septic systems for your ADU can be intricate and often requires professional guidance. Whether you’re considering utilizing an existing system, installing a new one, or connecting to a municipal sewer, it’s essential to stay informed and compliant with the latest regulations and environmental health standards. Keep in mind that the information provided here is an overview based on current practices. It may change as new technologies emerge and regulations evolve. Always consult with a qualified septic contractor or local regulatory agency to ensure that your project meets all necessary requirements. By doing so, you’ll be taking a significant step towards a successful and sustainable ADU project.
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