The biggest line item in a septic project is the system type, and your soil usually decides which one you're allowed to build. Here's how the three common choices compare.
Conventional gravity
The cheapest and most common system. Effluent flows by gravity from the tank to a drain field, where good soil does the final treatment. It needs land that percs in the ideal range and enough separation to the water table. Typical installed cost: $3,500–$10,500.
Mound systems
When the water table is high or soil is shallow over rock, a mound raises the drain field in engineered sand above grade. It needs a pump and more earthwork, which is why it's one of the priciest options — commonly $10,000–$25,000.
Aerobic treatment units
An ATU injects air to treat wastewater to a much higher standard before it reaches the soil, so it can work on difficult lots and small fields. It runs on electricity and needs a service contract. Installed cost is similar to a mound: $10,000–$25,000.
What drives the cost
- Soil & site difficulty — the main driver; poor soil forces a pricier system.
- Home size — more bedrooms, bigger tank and field.
- Region — local labor and permits swing the total 15–35%.
- Soft costs — perc test, engineered design, and permits add $1,000–$3,000.
Get the perc test first. The result determines which systems are even legal on your lot, which sets the realistic budget far more than any quote.