How a Septic System Is Designed and Permitted

The short answer

Design starts with a site and soil evaluation, such as a percolation (“perc”) test, that determines whether your lot supports a standard gravity system or needs an alternative type like a mound or aerobic treatment unit. System size is set mainly by bedroom count, not by how many people live there today. In Georgia, soil evaluations must be done by people certified by the state, and the system needs a county permit before anyone breaks ground. Check your own state’s rules.

Site and soil evaluation: the perc test

A perc test digs holes two to three feet deep, adds gravel, saturates the soil, and measures how fast the water drains at timed intervals. Per HomeAdvisor, a professional perc test typically costs $750 to $1,900, averaging around $1,300, and the whole process, including site prep and travel, usually takes half a day to a full day. A quarter-acre lot might need just one to four holes; a full acre can need anywhere from three to twenty.

Georgia’s rules call this a soil evaluation, so your paperwork may use that term instead of “perc test.”

Sizing: it’s about bedrooms, not people

Septic systems are sized by bedroom count, not current household size. Bob Vila notes that standard tanks start at 750 gallons for 1- and 2-bedroom homes. Penn State Extension’s sizing table lists minimums of 900 gallons for a 3-bedroom home, 1,250 gallons for 4 bedrooms, 1,400 for 5, and 1,550 for 6. Local codes set the minimum where you live, so your actual number depends on your state’s or county’s sizing table and your soil’s drainage rate, but bedroom count is always the starting input.

Conventional vs. alternative systems

A conventional system is a septic tank paired with a gravel or stone drain field that filters effluent through stone and soil. Chamber systems swap the gravel for connected plastic chambers, which EPA says are easier to deliver and build.

When the soil can’t support a conventional system, per the EPA’s system-types overview:

• Mound systems elevate the drain field in a built-up sand mound, used for shallow soil, high groundwater, or shallow bedrock.
• Aerobic treatment units (ATUs) inject oxygen to boost bacterial treatment, can be used on smaller lots or poor soil, and need ongoing maintenance for the life of the system.
• Drip distribution systems disperse effluent through drip lines laid in the top 6 to 12 inches of soil, but need a dose tank and electrical power.
• Recirculating sand filters provide extra treatment near sensitive water bodies or high water tables.

Which one you can install depends on your soil evaluation, and also on lot size, slope, home size, local and state rules, and budget, per EPA.

Who does the design

Who can evaluate the site and design the system depends on your state or county. In Georgia, for example, state rules require soil evaluations to be done by people certified by the Georgia Department of Public Health, and septic tank contractors must be certified too. Your county health department can tell you which licenses your state accepts.

Permit steps and fees

Expect to need a permit for a new system, a repair, or a modification, and permits typically only cover a fixed window before they expire; in Georgia, a construction permit stays valid for no more than twelve months. The state writes the rules, and the county health department (or a state environmental health office) usually issues the permit and inspects the work, so that office has the exact fee and requirements for your property. In Georgia, for example, the Department of Public Health writes the rules, but under the state rules permitting is handled locally by the county board of health.

State rules can also decide which design you’re allowed to build. Florida, for example, has a 2023 nitrogen-reduction law that requires new systems on lots of one acre or less in mapped “impacted areas” to use a nitrogen-reducing design, per the Florida Department of Environmental Protection. In four named planning areas of the Indian River Lagoon (the Banana River Lagoon, the North and Central Indian River Lagoon, and Mosquito Lagoon), commercial properties and homes on 10 acres or less with an existing septic system have to connect to central sewer where it’s available, or upgrade to a system that removes at least 65 percent of the nitrogen, by July 1, 2030, per Florida law and per DEP’s permitting page.

Permit fees are set by local city and health departments, so get a written quote from your evaluator or contractor before you commit to a design.

What happens after the design is approved

Approval isn’t the finish line. The county typically has to inspect the system during installation to confirm it was built to the approved design; Georgia, for example, bars anyone from backfilling or using a new system until the county board of health has made a final inspection and approved it in writing. Keep a copy of the approved design, the permit, and every inspection sign-off in the same place as your future pump-out receipts. It’s the same paper trail a buyer, an inspector, or a future contractor will ask for.

Work with a licensed designer

Soil, lot size, and local rules make every design different. A licensed septic design professional can run your site evaluation and walk your county’s permit process with you.

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