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How deep your septic system needs to be into the ground depends on several factors. In extremely cold environments, the tank needs to rest below the maximum frost line, which can be up to six or eight feet deep. The standard burying depth for most systems is between a few inches and four feet below ground level. Depth can also depend on the slope of the landscaping and other property variables. 

The best way to protect tree roots is to avoid the roots entirely by excavating outside the root radius of the tree. For nearby projects, it’s a good idea to construct a temporary retaining wall. But that’s not always possible, depending on the project and the root system. In these cases, consider using alternative equipment like an auger to tunnel under the ground.

Remember, tree roots can damage nearby cement foundations as they grow. If a tree is close enough to interfere with large excavations, it may be better to remove the tree than risk future problems.

There is no standard depth for burying a septic tank; they are typically buried anywhere from 4 inches to 4 feet underground. The final depth depends on several factors, including the depth of your main sewer line, the height of the bedrock, soil type, local regulations, and the property's landscape. If you plan to grow grass or plants over the tank, a depth of at least 4 to 6 inches is recommended. Deeper tanks can be more difficult to locate, but a septic professional can help find it for you.

If you don’t have a crawl space but have a partial basement you want to make deeper, you can expect to spend a little less—between $40,000 and $90,000 on average. However, if you’re not adding new finished square footage, you won’t see much in the way of ROI.