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Septic Systems for Accessory Dwelling Units (ADUs)

Accessory Dwelling Unit (ADU) Septic System

Hey, Brian Freeman, Freeman construction. Today I wanted to talk to you about septic systems for accessory dwelling units or ADUs. If you’re in San Diego County and you’re wanting to put an ADU or accessory dwelling unit on your property and you’re not on a city sewer, you don’t have access to city sewer, then this is something that you definitely have to pay attention to.

Land Layout & Usability

So when you are either buying property or considering putting ADU on existing property that you own, the first thing you need to look at if you do not have a large piece of property. This could be like an acre or less… Potentially a little bit larger depends on the way your lands laid out and the usability of your land for putting a septic system in. But you are gonna need to evaluate right from the very beginning if you can get a septic system on there for that ADU.

Seepage Pit or Leach Lines for ADU?

So we typically will start working immediately with a septic engineer on a problem property to establish the layout. But what happens is, you already have your primary residence and you have an existing primary system, the septic system, which is your leach field could be a potential seepage pit or it could be leach lines depends on the percolation rate. But that’s gonna cover a certain amount of area, the primary system that’s already in there, then the county of San Diego also requires you to have 100% reserve area. So even though there’s no system installed for your existing home, it’s there, you’re gonna need twice the space that you need. So if it’s 400 or 500 feet of leach lines, you’re gonna need to have additional area available for that another four to 500 feet and that’s so if your primary septic system fails, you have space to put in a new system.

Where to place & build ADU?

So with that in mind, once you have your site drawn out where your existing home is and that primary and reserve area laid out which a lot of times will be on file with the county, you can request a septic as built from the county and they will show you what is installed. Some properties are old enough and there’s not good enough records but for the most part you can find them pretty well. So once you have that, now we got to look at okay, well where are we gonna put the ADU on the property? How big is the ADU? ‘Cause you gonna have a 200 square foot ADU or up to that for detached but you also can add a garage and there’s not a limitation on the garage size typically. So that’s the footprint of that ADU of the foundation can take up a significant chunk of a smaller lot.

Septic Systems & Wells

So we got to look at where can we actually build that and how big that’s gonna be and then we got to take into account that we need to have a typically. It’s a two or three bedroom that based on the bedroom size for that ADU, how much leach field or how much area for the seepage pit we’re gonna need to have for the primer and then also hundred percent reserve area as well. And then you got to put that along the septic layout and make sure all that’s going to fit. So the other thing that you have to consider too is if you’re on a well, which a lot of these properties are that are on septics is, you have to have 100 foot setback from the septic system that you’re putting in, the reserve and the primary to the actual water well that’s drilled. So that takes 100 foot radius of wherever that well’s at that eliminates where you can put your septic system. So pretty important as far as looking at that on the front end. You don’t want to draw your accessory dwelling unit plans or ADU plans and then go submit that down to the county and then find out, no you can’t even do it because you have don’t septic space.

Percolation Testing

So another thing to consider is you need to make sure that you know the percolation rate for the existing septic system that you have in. What was the testing down the soil, basically how they fill up a hole with water and how long does it take for that water to go down every inch, so they measure that in amount of minutes. So if you’re under 30, you can have a seepage pit, if you’re above 30 you’re gonna be leach lines, seepage pit typically takes a little bit less, it’s gonna take less area to put a seepage pit in, but you also have to consider the groundwater so if you have high groundwater, seepage pits are not gonna work.

Alternative Septic System

 So, okay and then one additional thing that you can do so if we have a limited area or we have high groundwater as far as getting a septic system in for an ADU is we can put an alternative septic system in or an ATU and that will allow us to. They’re only like six inches deep that the system is versus going a lot deeper for a standard system. So we can put those in high groundwater areas and they take up less space typically. You’re able to maybe fit show that as the reserve area, taking up less space and then we’re able to get that ADU to fit on the property. So there’s a of variables.

Septic System Setback Requirement

The other thing, and we’re actually working on this project right here, right now. This has an existing home on it, that they took to turn into ADU and they’re gonna put a new home on this property. This one has a flow line where the water comes through off of that flow line, where the water’s at, there’s a 50 foot setback required. This particular ditch I think is too large where we can’t actually put concrete in this ditch to line it. So we end up having to stay 50 feet off of that for a leach field, which significantly impacts this lot and really pushes this house into a small area that you can build on. So, another variable having the water flow areas can restrict where you put your septic system.

Septic Systems for Accessory Dwelling Units (ADUs)

That is a quick overview of why you need to investigate septic systems on the very front end when you’re installing an accessory dwelling unit or an ADU on your property. So make sure you do that. If you have more questions about that, we can help you. Feel free to reach out to us. Great talking to you and have an excellent rest of your day.

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