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

Alternative Septic System (ATU) for Accessory Dwelling Units

We’re going to talk about how you can use an alternative septic system to get a septic system in for an accessory dwelling unit that’s on a small lot because on a small lot it’s very tight and doesn’t have much space. An alternative septic system, a lot of times you’ll hear it called an ATU, is an Aerobic Treatment Unit, so don’t confuse that with an ADU, which is an Accessory Dwelling Unit.

Primary Septic System | Main Home

On property that already has a primary dwelling on it, so your main home, that home will have had a primary septic system. When you get approved with the Health Department, the City or the County Health Department, what has to happen is you not only have to have enough space for your primary septic system, but you also have to have 100% reserve area. The reason being if that system was to fail 30, 40 years down the road from when it’s installed, you have to be able to put another system in, which would go in this area that’s designated for the reserve area.

Figuring Out Septic Area for a Standard Septic System

When you put an Accessory Dwelling Unit on a property, what happens a lot of times, you will impact the area that is either for the primary, or for the reserve area.  That creates a problem for getting a septic approval with the County of San Diego. So, what we have to do is get a septic engineer involved. They will:

  • Drone photos of the property.
  • Surveys if we don’t have corners marked already.

Which then allows us to:

  • Locate the primary septic system that’s for the main house.

Then, we will find out how much additional septic area that we need to put a standard system in for this Accessory Dwelling Unit, depending on the amount of bedrooms that the ADU has.

When to Use an Alternative Septic System

Once we know how big the system has to be and how many lineal feet of leach lines that it would require, we then map that out on the parcel map, which is the aerial that’s taken by this drone, that shows where this would go. Now, even if we have enough room to have a:

  • Primary system for the ADU.
  • Primary system for the main home.

We lose the reserve area for the primary dwelling a lot of times, and we don’t have it at all for the ADU, if we were going to figure on putting in another standard system. This is where the ATU, the Alternative Septic System comes in as a very handy tool that we can use in this situation.

Septic System Costs | Alternative vs. Standard

The cost for an alternative system versus a primary standard septic system is pretty significant. If you’re putting in a standard system, you’re going to spend somewhere between $10 to $15,000, typically. But if you are going to put in an alternative system, you’re going to spend somewhere to $30 to $50,000, depending on the size of that system that you need for the house. So, what that means is we don’t like to use the alternative septic systems when we don’t have to, so we can save money during the build by not doing that.

Alternative System for the Reserve Area

What we’ll use it for, is for the reserve area instead, so the alternative system will get designated as being used only in the reserve area. What that means is that we can put in a primary system, or a standard system for the ADU, and we can either:

  • Get that added to the existing septic system.
  • Put in a new one for that particular ADU depending on the situation on the lot.

It allows us to not spend the money on an alternative septic system because we can just show it on the septic layout, that it is designated for an alternative septic system in the reserve area, if it’s needed. So, it’s not actually installed, it’s just shown that it’s feasible to do, and It takes up a lot less area.

Percolation Testing Completed by the Septic Engineer

It’s huge, as far as allowing us to get an ADU on a property that you would otherwise not be able to do. So, very helpful. And, part of that process of getting the alternative treatment system approved, is we actually have to do a percolation test that they will do in the area that the ATU will go. So, that’s all part of the process. When you get our septic layout done by the septic engineer, he’s going to test that area, and then will  get that submitted along with the layout to the County, which allows them to get that approved.

More Resources for Home Projects!

That’s why and how you would use an alternative septic system for an ADU on a tight lot. I hope that helps. If you would like more information on building ADU, building a custom home, remodeling a home, or you want floor plans for ADUs, floor plans for custom homes, you can get it here on our website at tfgonline.com. It was awesome talking to you today, and have a great rest of your week.

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