Septic Soil Evaluation in Texas: How It Works, Cost, Who Does It

What is a septic soil and site evaluation?

A soil and site evaluation is the on-the-ground study that tells the county what kind of septic system your lot can safely handle, and how big it has to be. Texas requires one on every tract where a septic system will be installed, under 30 TAC 285.30. No evaluation, no design. No design, no permit.^source

It has two halves. The soil half looks at what is under the grass: texture, layers, anything that blocks water, and signs that groundwater comes up close to the surface. The site half looks at the lot itself: slope, low spots, the 100-year floodplain, and every well, pond, creek, property line, and building the system has to stay away from.

The rule is blunt about what happens next: the information from the evaluation is used to determine the type and size of the system. That is why this report matters more than any quote. Every installer's price is built on it.

Is a perc test still required in Texas?

No. Texas no longer designs septic systems from a percolation test. TCEQ says that before February 4, 1997, most systems in the state were designed from perc tests, and that those tests did not address limiting factors to two feet below the drainfield or the soil texture in the disposal area.

The current rule replaced the stopwatch-in-a-hole approach with a soil profile. The evaluator digs down, reads the layers, and assigns the soil to a texture class. Each class carries a loading rate in the rule tables, and that rate, not a timed drop in water level, sizes the field.

You will still see "perc test" used loosely by sellers, agents, and even some contractors to mean the whole evaluation. That is fine as shorthand. Just know that if someone offers you a perc test result from decades ago, it does not answer today's question. A system permitted now needs an evaluation done to the current rule.^source

Who can do a soil and site evaluation in Kaufman County?

Statewide, a site evaluation must be done by a TCEQ-licensed site evaluator or a professional engineer. Kaufman County is stricter. Its TCEQ-approved order says only a registered professional engineer (PE) or a registered professional sanitarian (RS) with a current site evaluator certification will perform site evaluations and complete septic designs, as-builts, or system certifications.^source

In practice, that means the person evaluating your soil is usually the same person designing your system. Many installers have a PE or RS they work with and will line up the evaluation for you. That is normal. Just know who it is and ask for a copy of the report in your own name.

You can check any Texas septic license on TCEQ's license search. Our guide to who can work on a septic system in Texas explains the site evaluator, installer, and maintenance provider licenses.^source

How much does a septic soil evaluation cost?

In Kaufman County, plan on about $350 to $700 for the soil and site evaluation. Because the county requires the same class of professional to do the evaluation and the design, it is often quoted as a package, so ask whether your number covers the report alone or the report plus the design.^source

What moves the price:

• Auger borings vs backhoe pits. Pits show more, especially in stiff clay, but need a machine on site.^source
• Access. Brush, fences, a locked gate, or wet ground can mean a second trip.
• More holes. The rule sets two as the minimum, and the permitting authority can require more where the soil varies.^source
• Survey and floodplain work. If there is no recent survey or the lot touches a floodplain, expect more drafting time.
• Bundled design. A package that includes the stamped system design costs more than the evaluation alone, and is usually the better value.

What happens during a site evaluation, step by step

Most evaluations are one site visit plus office time to write the report. Here is the sequence the rule lays out, in the order you will usually see it.

• Pick the likely disposal area. The evaluator looks at where the house, driveway, and any shop will go, then chooses where the drainfield, spray area, or drip field is most likely to fit.
• Dig two holes at opposite ends of that area. These are soil borings (hand or truck auger) or backhoe pits. They go two feet below the planned excavation depth, or down to a restrictive layer, whichever comes first.
• Classify the soil texture in each hole. The evaluator assigns a Texas soil class, from Ia (gravelly sand) to IV (clay).^source
• Run a gravel analysis if needed. Class II or III soils with gravel get a sieve analysis to measure how much gravel is in them and how big it is.
• Look for restrictive horizons. Clay subsoils, rock, and plugged laminar soils that an auger will not go through all count.^source
• Check for groundwater. The evaluator examines the soil profile for signs of groundwater within 24 inches of the bottom of the planned excavation.
• Map the surface. Slopes, depressions, gullies, and the 100-year floodplain from FEMA maps all go on the site drawing.
• Measure separations. Wells, water lines, ponds, creeks, property lines, foundations, and pools are located so the design can meet the Texas setback table.
• Write the report. The findings and a site drawing with every hole location go in with the permit application.^source

What the soil evaluation report tells you

The report is the most useful piece of paper in your whole septic budget. Each finding narrows which systems are legal on your lot and how much ground they need. Here is how to read it.

Texas soil classes and what each one allows

Texas sorts soil into five classes, and the class is the single biggest factor in your system type. The rule lists them by texture: Class Ia is sandy soil with more than 30% gravel, Class Ib is sand and loamy sand, Class II is sandy loam and loam, Class III is silt, silt loam, silty clay loam, clay loam, sandy clay loam, and sandy clay, and Class IV is silty clay and clay.^source

For a conventional drainfield, the class sets the loading rate: roughly 0.38 gallons per square foot per day for Class Ib, 0.25 for Class II, and 0.20 for Class III. Divide your home's daily design flow by that rate and you get the absorptive area. A 3-bedroom house at 240 gallons per day needs about 1,200 square feet in Class III soil. Our septic system size guide has the full chart.

Classes Ia and IV sit at the two ends and are both rated unsuitable for standard subsurface absorption. Ia passes water too fast for the soil to treat it. Class IV barely drains at all. On Class IV clay, drip after aerobic treatment is allowed at 0.1 gallons per square foot per day, and a pumped-effluent drainfield is possible on single-family lots with slopes of 2% or less.^source

How Blackland Prairie clay results usually play out

On most Kaufman County lots, the evaluator hits clay, writes down Class IV, and the design becomes an aerobic system with spray or drip disposal. The Blackland Prairie soils that make this good farmland (Houston Black clay, the official Texas state soil, is the classic example) shrink and crack when dry, swell when wet, and pass water very slowly.^source

TCEQ's 2024 permit data shows how consistent that is: 514 permits issued by Kaufman County, 488 surface irrigation (spray), 14 drip, 5 low-pressure dosing, 7 other, and no standard trench systems. When your report comes back Class IV, you are in the same boat as nearly every neighbor who built recently.^source

Two nuances are worth knowing. First, some tracts have a few inches to a couple of feet of loamy topsoil over clay subsoil. The clay underneath counts as a restrictive horizon, so the loamy top does not rescue a trench design. Second, clay reads differently wet and dry. A good evaluator looks at color and mottling in the profile for signs of a seasonal water table, not just whether the hole is wet on the day.^source

What if you disagree with the result?

The rule has a built-in tiebreaker for groundwater, which is the finding people most often dispute. If the county's designated representative and your evaluator disagree about groundwater, the county checks the NRCS soil survey for the county. If anyone still disagrees with the survey, the owner can hire a certified professional soil scientist to evaluate it and present that to the county for a final decision.

Soil texture disputes are rarer, because clay is not subtle. If a second professional reads the same holes differently, ask both to explain what they saw, in writing. A second opinion is fair. Shopping for an evaluator who will write down a better soil class is not, and the county inspector will see the ground when the system goes in.^source

If the problem is space rather than soil, a different disposal method can help. Drip can fit tighter lots than spray. Our spray vs drip comparison explains the trade-offs.

When to get the evaluation: before you buy

Get the soil and site evaluation before you close on land if the seller will allow it, or make your contract contingent on it. It turns the biggest septic unknown into a number before you own the problem.^source

Kaufman County also requires a minimum of usable space: one acre on lots in public-water subdivisions platted after January 1, 1988, and two acres on new lots with a private well. The evaluation and site drawing show whether the system and its setbacks actually fit on that usable area. The 1-acre usable space rule explains what counts.^source

When you are ready to price the system itself, our septic system installation page connects you with licensed installers who work from your report.^source