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What are Septic Perc Rates, Septic Layouts & Septic Asbuilts?

Septic Systems & Percolation Testing in San Diego County

We are going to cover septic systems and will talk about:

  • Septic Layouts
  • Percolation Rates
  • Septic as Built

And will cover specifically:  

  • What the difference is between those three things.
  • How they pertain to purchasing land or the build process.
  • What you need to know about them.
  • How that process works.

Percolation Testing & Perc Rates

The first thing if you’re looking for a piece of land and nothing has ever been built on it you want to look and see if there’s been a perc test done. A percolation test is essentially how fast the water soaks into the ground. We’re going to look at the percolation rate, they measure it in inches per minute. The septic engineer will drill a hole and put water in that hole to test:

  • How it recedes and soaks into the ground.
  • The amount of time it takes for the water to recede and soak into the ground.

That’s your percolation rate. The faster it goes into the ground, the better the percolation rate, the less time it takes to go into the ground which means the septic system that you put in does not have to be as large because the smaller system will do a same amount of work as a larger system would with a higher perc rate. That’s the basics of how percolation rate works.

How Much Does a Percolation Test Cost?

A percolation test may cost somewhere in the neighborhood of $3,500 so it may or may not make sense depending on the system that you’re putting in and what that cost will be.

How to Save on Percolation Testing

Let’s say you have a piece of property that was subdivided, it’s a legal lot and there’s lots around it that have been built on, now, that particular property that you’re buying may have not have had a percolation test done on it, but with the County of San Diego, what you’re able to do a lot of times is you can contact them during the septic layout process and say, “Hey, look, these properties around me have had perc test done on them, they’re similar soils,” and they will look at what the perc rates were on those properties around you, the adjoining properties, and they’ll take the worst case perc rate. 


Say one percolation rate is 40 and one is 20, they’re going to take the 40 and then allow you to move forward, typically, with a system designed for a perc rate of 40. By doing this you will not have to worry about getting a percolation test done. 

When Should I Do the Perc Test?

The process of the septic layout can occur while you’re actually have your plans for your home at the County of San Diego getting processed or City of San Diego wherever you’re at, and you can do those simultaneously, however, you can’t get a permit issued to build a home without that septic layout actually approved first and then pulling that, you’ll pull that permit of the septic at the same time.

Different Septic Systems

There are different types of systems. You can install a:

  • Alternative Septic System
  • Standard System
  • Seepage Pit System 

Alternative Septic System

Let’s say you do a perc test and you find that there’s groundwater that’s not very far below the surface and you have to have a separation between the groundwater and the effluent that’s coming out of your system. In this case, you would want to use an alternative system because of the potential to have high groundwater and that can affect the cost because you’re going to spend a lot more on an alternative system than you would on a standard septic system. Costs depend on how many square feet you need.

Costs = $30,000 to $45,000

Standard Septic System

On a standard system, you will perform a perc test. After determining that you don’t have the potential for high groundwater you can go ahead and draw up what we would call a septic layout. This would be on a piece of property that has not, again, been developed, has never had anything on it before. 

Costs = $10,000 to $15,000

Build Permit Qualifications

The septic layout is essential to get done so you can actually start that process before you start to get a building permit. The septic layout is a drawn up site plan, a lot of times it’s on 11 x 17 sheet that you will show: 

  • Property Boundaries 
  • Location Of The Home
  • Any Grading Areas

Areas that are being graded, because that septic system cannot go in an area that has been impacted by grading. They want that in the natural soil. Once we have the locations for the:

  • House
  • Pad
  • Grading

We’re going to show where the primary septic system is going to go and then we’ll show the reserve area as well.

Reserve Area Requirements for Septic Systems

Standard systems, you’re going to be required to show a hundred percent reserve area and the same for alternative systems. However, the standard systems typically take a lot more space than an alternative system. All of that will go on the septic layout, you’ll show any:

  • Easements
  • Anything else that potentially impacts where a potential septic system could go.

Then you’re going to get that submitted to the Department of Environmental Health and once that’s done, they will say, “Hey yeah, I like it,” or they’ll tell you what changes need to be done on the system, makes recommendations, you make those changes and then they’ll approve that septic layout.

How Bedrooms Can Affect Septic System Approval

One thing that affects the size of the septic system that you’re trying to get approved is the amount of bedrooms. It is NOT:

It’s the amount of bedrooms. So if I have a three-bedroom home, I’m going to have to have a certain amount of, or a certain size septic system, so, a certain amount of leach line, a footage of leach lines. If I go to a four-bedroom, I would have to have a certain amount more leach lines for that fourth bedroom.

How to Avoid Having to Install Extra Leach Lines

A lot of times if you’re approved for a three-bedroom, they’re going to let you go, they’ll just tell you, “Yeah, well, you’re a three-bedroom at this percolation rate at a hundred linear feet of leach lines,” or whatever that calculation is for the fourth bedroom. Even though it was perc tested for a three-bedroom initially, you can no problem do a four, but that doesn’t always occur if you’re limited on space. 

If you ran out of space for your primary septic system and your reserve, then you may not be able to do that. You could be limited to a two or three-bedroom, or some of these ADU’s we’re doing are limited to one-bedroom because there’s not enough space on the lot to put that extra bedroom and extra leach field that’s required. 

Septic As Built | Different Septic Systems

What is a septic as-built? A septic as-built is different, an as-built is if you’ve had a system put in on the property, the septic contractor, going back to the layout, has the plan for the basic layout, now, once that has been approved, you’re going to start the work. The system gets installed and the septic contractor is responsible for drawing up a septic as-built and that’s what is actually installed. A lot of times, it’s exactly as the septic layout is, but sometimes you’ll hit rock or there’s something unforeseen where you have to move the system, or maybe one line is longer than it was shown on the layout. In this case you’ll actually draw the exact lineal footage of the leach field on each individual line and put that on this as-built. It’s as it was built, that’s what an as-built is. The layout is prior to the work actually being done.

How to Access Septic As Built Information

Sometimes, you’ll have a property where the home was either:

  • Torn Down
  • Burned Down

When you’re buying land and know that there is a home there or a home on the property prior, you can pull up that septic as-built in the county records and you can see what’s actually installed and where because you’re required to put dimensions to show where it was located on the property which helps the future owner of that property to locate that if need be. It should show where the septic tank is at and exactly where all of your leech field are. Keep in mind however, it’s not always as it was put in, how it’s drawn, especially as you get older properties, but it does give you a starting point on where to find that.

Freeman’s Construction Inc Helpful Resources

If you have more questions you can request a consultation or give us a call. Today, we covered:

  • Septic Systems
  • Septic Layouts
  • Percolation Tests
  • What a Septic As Built Is

You also can download a ton of information on:

Thank you for your time, have a great rest of your day.

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