ADU Septic Systems 101

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 granny flat. 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.

What are different types of septic systems and how much do they 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. 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.

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.

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?

It depends. 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 contractor and examine what your options are.

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.

There is preliminary work that must be done before submitting a septic plan, which most notably includes a feasibility study to determine whether or not septic is even an option in your area. 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. This is an incredibly 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.

It is worth looking into which alternatives will serve your needs and location best by doing a septic feasibility study. This first step will direct you towards the system that works best with your budget and property and makes construction as seamless as possible.