Septic System for an ADU: What You Need to Know

Nov 10, 2023 | 0 comments

Determining whether or not your accessory dwelling unit 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 that 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.

No matter the style of septic system you choose to go with, each one 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, as 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 all ADUs need a septic system?

No! Septic systems are only necessary if your property is not connected to a city sewer system. 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,” 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.
  • Area Allocation: For a basic septic system and required reserves, you’ll need approximately 5,000 square feet of usable land. This area should 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. These setbacks, often around 10 feet, are essential to prevent contamination and ensure the system functions properly. They must be maintained from existing structures, property boundaries, water bodies, and even other septic fields.

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.

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.

The process typically involves several steps:

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. 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.

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.

How long does it take to add a septic system?

You will first need your septic system approved by the Department of Environmental Health in your area. After submitting your septic plan, it can take about 60 to 90 business days to get it approved. After this, construction takes about two weeks. The system is then tied into your ADU’s plumbing, which completes the final step.

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. Here’s what they will consider:

  1. 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.
  2. 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.
  3. 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.

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.

What are different types of septic systems and how much do septic systems 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 residents in other large cities 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

Because it is most common for septic tanks to connect to a leach field, this system is often referred to as the “traditional septic system.” 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, averaging 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, which means this system is not a viable option for all property owners. Certain lots present challenges for a traditional leach field. In those situations, there are alternative septic seepage systems that can be utilized.

Vertical Seepage Pit

A vertical seepage pit acts in a similar manner to a leach field, although 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, which can be quite extensive in size. 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

These pits are very similar to their vertical counterparts, so little additional explanation is needed. 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, costing 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 based on its size and bedroom count.

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 and 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 and is executed safely and legally. By doing so, you’ll be taking a significant step towards a successful and sustainable ADU project.

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