Septic system installation runs $6,000-$30,000 in 2026 depending on soil conditions and system type. Conventional gravity-fed systems land $6,000-$15,000 when soil percolation is adequate. Advanced treatment systems (ATU, aerobic, sand filter) run $15,000-$30,000 in poor-soil or environmentally-sensitive areas. Mound systems for high water tables: $12,000-$25,000.
Perc test first, design second
Percolation test ($300-$900) determines how fast water absorbs into your soil, which drives every other system decision. Tests typically dig 3-5 holes to the proposed drain field depth, saturate them, and measure drop rate. Results dictate: system type (gravity vs pressure), drain field size (poor perc requires larger fields), tank size (higher water use flags with bad soil), and whether a conventional system is even allowed.
Tank sizing by household
Minimum tank size by bedroom count (universal code): 1-2 bedrooms 750 gallons, 3 bedrooms 1,000 gallons, 4 bedrooms 1,250 gallons, 5+ bedrooms 1,500 gallons. Some jurisdictions increase these for gray-water-heavy households (hot tubs, multiple washing machines). Concrete tanks ($1,200-$2,500) are the dominant choice and last 30-40 years; plastic tanks ($800-$1,800) are cheaper and lighter but easier to crush from above-ground equipment.
Pump systems and advanced treatment
Gravity systems work where the drain field is downhill from the tank. Pump systems (required when drain field is uphill or distant) add a pump chamber and effluent pump — $1,500-$4,000 extra. Advanced treatment units (required in poor-soil or environmentally-sensitive areas) add aeration, sand filtration, or denitrification — $8,000-$20,000 extra. ATUs require annual maintenance contracts ($150-$450/year) to maintain permit compliance.