Septic Tank and Drain Field Considerations

When I previously wrote about selecting a lot for building a house, I mostly discussed personal preferences. Here, I’ll be focusing on another big piece of the lot selection puzzle… The suitability of the land for a septic system.

As a reminder, the neighborhood where we’re building isn’t connected to a municipal sewer system. As such, we’ll be depending on a septic tank and drain field for treating our wastewater.

Septic system design

Ideally, this system would be as simple as possible, depending entirely on gravity to move waste out of our home. The alternative would be using a sewage ejector to get waste up and out of the basement or, even worse, a secondary tank and pump to move wastewater up to a higher drain field.

The disadvantages of a sewage ejector or septic pump are obvious. If the ejector or pump breaks down, the float gets stuck, etc., the system will be crippled. Likewise, these things depend on electricity, so problems could arise in the event of an extended power outage.

We also want to stick with conventional septic drain lines in gravel trenches as opposed to having to use a more costly (and less desirable) “drip” system to dispose of the wastewater. This mainly depends on soil quality.

Our initial lot selection

We initially chose a lot that backs onto 3.5 acres of forested common space. Lots on this stretch slope up from the street, but flatten out nicely in the back half. The first lot that we picked out also had a fair bit of drop from right to left, which meant we could have a basement that opens to the left.

The downside of this lot was that the soil survey indicated a sizable patch of bad soil with fairly shallow bedrock right in the middle of the lot. The builder wasn’t concerned about sinking a foundation into this soil, but it did create some potential issues with septic placement.

This particular lot was also classified as being “drip repair.” This means that, if the original septic system were to ever fail, it would likely need to be replaced by a drip septic system. Not great, but not a showstopper since complete septic failure is relatively uncommon.

In looking at this lot, we hoped that we could gravity flow the system out of the basement. In this case, everything would drain down to the main line in the basement, the septic tank would be placed at the front corner of the house, and the drain field would be located in the front yard.

We made an offer and got that lot under contract, but with a contingency regarding suitable positioning of the house and septic system. We then consulted with the engineer who had designed the subdivision.

The engineer expressed concern about fitting our desired floor plan on the lot we had selected while leaving sufficient space for a traditional septic system. He said we might be able to fit a 4-bedroom system on that lot, but it would be tight. And a 5-bedroom system was probably out of the question.

Side note: Septic systems are sized based on the number of bedrooms in the house. The more bedrooms that you have, the larger the system needs to be.

This 4-bedroom restriction wasn’t a huge deal, as the floor plan that we like best has an “optional 5th bedroom” that can be easily left as an office or nursery. The bigger issues were that we’d:

  1. likely need to put the tank out back with a sewage ejector to pump waste out of the basement and up to the main drain line,
  2. probably have to split the drain field between the back and front yards to get enough drain line in suitable soil, and
  3. definitely have to do more extensive soil testing (Level 4 Soil Report) and have a site plan drawn up to satisfy the health department.

The cost of all of this? Around $1,500 plus another $300-ish for the septic permit application. And the best case scenario was a more complicated system than we wanted to deal with, along with the unknown of possibly have to do a drip system if we run into trouble in the future.

And if the soil tests didn’t work out as we hoped? Well, we’d be out all that money with nothing to show for it… Hmmm, no thanks. But with that house/septic placement contingency in place, it was easy enough to back out of our original contract and look elsewhere.

Looking down the street

Disappointed but undaunted, we pulled out the plat map and started to (re)consider our options. A number of the lots along that stretch are too steep in the front for our taste but, as we moved down the street, we found another great option that backs onto the same piece of common space.

This new lot had a better overall shape (rectangular vs. sorta trapezoidal, which gives use more flexibility in siting the house), much better soils, and it still had some of the same right-to-left drop off, making it possible to have a walkout basement opening to the left.

Another advantage that had originally escaped us was that this lot is bordered on the left side by a 20 foot strip of forested green space. Thus, we can slide the house a little further in that direction while still maintaining some distance from the house that eventually gets built next door.

We asked the builder to take another look and he agreed that it would nicely accommodate our preferred floor plan, complete with the walkout basement, It also flattens out nicely in the back, and the soil survey indicated that we’d (probably) be able to use the simplest possible septic design.

In short, everything will drain down to the main line in the basement and flow out to the septic tank at the front left corner of the house. From there, the wastewater will flow passively out of the septic tank and into the drain field, which will be positioned in the front yard.

Satisfied, we put that second lot under contract with the same contingency re: house/septic positioning and submitted our permit request to the Health Department. We should have an answer within the next 5-7 days and will then (hopefully) be free to move forward with the process.

Update: Our septic permit request was approved.

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