Menu Close

How to Install a Septic System for an Accessory Dwelling Unit (ADU) on a Small Lot

How to Install a Septic System for an Accessory Dwelling Unit (ADU) on a Small Lot

Today I want to review how to use an alternative septic system for an Accessory Dwelling Unit on smaller properties that do not have enough room for traditional septic systems. People often refer to an alternative septic system as an ATU, which stands for Aerobic Treatment Unit. Do not confuse this with an ADU, which is an Accessory Dwelling Unit.

Primary dwellings have their own primary septic tied into them. When planning to build an ADU, getting approval from the Health Department is important. This can be in the City or County. To install an additional septic system, there must be enough space and 100% reserve area is required. The reserve is important in case that system fails years down the road. Let’s say in 30 or 40 years, if a septic system fails and a new system is needed, that new system can be installed in the designated reserve area.

Making the ADU Fit

Many times, when planning to build an Accessory Dwelling Unit, the property tends to be a tight lot and the new ADU footprint will impact either the primary or the reserve area. This creates a problem for getting septic approval with the County of San Diego. In this case, we must involve a septic engineer who will take drone photos of the property, survey the property if no corners are already marked, perform a percolation test, and locate the primary house system. We also need to find out how much additional septic area is needed for the new Accessory Dwelling Unit, which depends on the number of bedrooms in the ADU.

Once we know the required septic size for the ADU, we can draw that out on the parcel map and determine where to install this system. Now, even though we have enough room for the new ADU primary septic, and we can even keep the main house septic, if anything impedes on the existing reserve or the needed reserve, and the ADU comes up short of the total lot space needed to add the new structure, this is when the ATU, or Alternative Septic System, comes in very handy.

Additional Costs

The cost for an alternative system versus a standard septic system is significant. A standard system typically costs between $10,000 to $15,000. An alternative system typically costs between $30,000 to $50,000 depending on the necessary size that is required for the house. The increased cost deters us from using alternative systems when we don’t have to. Even though it is required, often we design the reserve area as the alternative system, and add the primary system as a traditional system.

A Few Options

This allows us to choose from a few different options. We can either add on to the existing system, we can install a new primary septic system for the ADU, or we can use the ATU system. Due to the high ATU price, we often design the reserve area as an alternative system, which saves us from spending the money on the system because the only requirement for the reserve area is to draw the feasible designation on the septic layout. This saves a lot of space since an ATU takes significantly less square footage than a standard system.

An ATU option is a huge game changer when it comes to allowing an ADU on a small property, and these are the reasons why and how an alternative septic system is used. I hope that helps. If you would like more information on building an ADU, a custom home, remodeling a home, or if you want floor plans for ADUs and homes, you can get those here on our website at www.freemansconstruction.com. Have a great rest of your week.

If you would like to partner with us and be a guest on “The Real Property Show” please go to realpropertytv.com and submit a request.



1. Request a Project Consultation Online or by Phone

2. We Will Meet With You at Your Home or Building Site

3. Establish a Budget for the Project & Start Drawing Plans

Related Posts

Leave a Reply

Your email address will not be published. Required fields are marked *

Scroll to Top