What Is the Average Cost of Septic System Installation in 2026? The average cost of septic system installation sits at around $8,000 nationally, but what you'll actually pay depends heavily on your system type, soil conditions, and location. Here's a quick snapshot to answer your question right away: System Type National Average Typical Range Conventional gravity system ~$8,000 $3,000 – $15,000 Aerobic treatment unit (ATU) ~$15,000 $10,000 – $20,000 Mound system ~$15,000 $10,