Most residential basements are 8 to 10 feet deep. You can dig deeper depending on building codes, but it does become more difficult. You’ll likely need to install stronger foundation walls and additional supports to handle the increased load. Depending on the water table, you may also need a water management system. Clay soil can support a deeper foundation than sandy soil. A structural engineer will be able to tell you the maximum, feasible depth.

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.

Most states do not require excavation companies to hold specific licenses, but many require at least a contractor’s license. Additionally, heavy equipment operators require a special commercial equipment license to operate machinery required for excavation. You can use Angi’s contractor licensing tool to confirm that an excavation company you’re considering hiring is fully licensed. You should also make sure that the excavation team is fully insured before allowing them to work on your property.

You can dig 60 to 80 linear feet by hand per day depending on the depth. Digging a 100-foot trench by hand will take you two work days, but it can take longer if you have heavy or rocky soil. If you’re digging a deep trench or one that’s over 100 feet, consider renting a mechanical trencher, such as a walk-behind or ride-on, or calling a pro for help.