Texas has not sized a septic system on a percolation test since February 1997. A Texas permit requires a site evaluation under 30 TAC §285.30, done by a TCEQ-licensed site evaluator or professional engineer, who digs two borings or backhoe pits, classifies the soil by texture, and looks for rock, clay, and groundwater within two feet of where the drainfield would sit. One Central Texas installer publishes $300 to $600 for the evaluation alone and $500 to $1,200 with the engineering; no county charges for it. Land does not “fail” the evaluation: clay rules out a standard trench and routes the lot to a different system type.
Since February 4, 1997, Texas has replaced the timed water test with a soil profile read by a licensed evaluator, and the current rule contains no percolation step at all. TCEQ’s own guidance to owners says why: “Before February 4, 1997, the most common way an OSSF was designed was based on percolation tests. Unfortunately, the percolation test did not address any limiting factors to a depth of two feet below the bottom of the drainfield or address the soil texture in the disposal area.” The phrase survived the rule change in listings, lender emails, and installers’ own copy; the procedure did not.
Under §285.30(b)(1), the evaluator “shall either drill two soil borings or excavate two backhoe pits at opposite ends of the proposed disposal area,” dug “to a depth of two feet below the adopted excavation of the disposal area, or to a restrictive horizon, whichever is less.” From those holes, the evaluator records three things. The soil’s texture class, from Class Ia (gravelly sand) through Ib (sand), II (sandy loam and loam), and III (silt loams and clay loams) to Class IV (silty clay and clay). Whether a restrictive horizon is present: “Clay subsoils, rock, and plugged laminar soils are considered restrictive horizons,” recognized as “a clayey subsoil which an auger will not penetrate” or “rock-like material which an auger will not penetrate.” And whether there are “indications of groundwater within 24 inches of the bottom of the excavation.” Nobody fills a hole with water and times it. The texture class then sets the loading rate that sizes the field, as the drain field sizing guide shows table by table.
Only a TCEQ-licensed site evaluator or a Texas professional engineer may perform the evaluation, and that holds even for a homeowner installing a system on their own land. Section 285.30(a) requires it “on every tract of land where an OSSF will be installed,” and the owner-installer exemption in 30 TAC §30.244 keeps the requirement: “The site evaluation must be performed by an individual who possesses either a current site evaluator or a professional engineer license.” An installer’s license alone does not qualify; TCEQ lists Installer II as one of six prerequisites for the evaluator license (with designated representative, PE, registered sanitarian, certified professional soil scientist and geoscientist in soil science), and every applicant must pass the Soil Analysis Exam.
Some counties narrow the field. Kaufman County’s Order allows “Only a registered professional engineer or registered professional sanitarian (with a current site-evaluator’s certification)” to evaluate and design; Medina County requires a design by “a licensed Site Evaluator, Registered Sanitarian with a site evaluators license or licensed Professional Engineer”; on the Edwards Aquifer recharge zone, §285.40 requires an engineer or professional sanitarian. Check the county’s permit authority before hiring; a report from the wrong license class will not be accepted.
One Texas installer publishes figures: $300 to $600 for the evaluation by itself and $500 to $1,200 for evaluation plus engineering, and no county fee applies to the evaluation. No evaluator or design firm found by this page prints a price; each asks for a call. A second installer’s guide carried $600 to $1,000 with design when this page was drafted and dropped the figures in a rewrite the next day, so they are not printed.
| Source | Figure | What it covers | Date |
|---|---|---|---|
| MAC Septic, San Marcos | $300–$600 | Site evaluation, Hill Country counties | March 2026 |
| MAC Septic, same guide | $500–$1,200 | Site evaluation and engineering | March 2026 |
| Angi (national) | $750–$1,900, average about $1,300 | Percolation test, timed water method | September 2026 |
| HomeGuide (national) | $200–$1,500 hand-dug; $500–$3,000+ by excavator | Percolation test | January 2024 |
| Fixr (national) | $600–$800, average $725 | Percolation test | January 2025 |
National estimators price a different procedure: the timed water test Texas retired, and their per-hole pricing does not map onto a two-boring evaluation. The Texas figures are one installer’s published ranges, not evaluator quotes; treat them as a bracket and get two or three written quotes from license holders. The fee schedules for Comal, Kerr and Guadalupe Counties carry no site-evaluation line at all: the evaluation is a private purchase on top of the permit fee.
Texas has no fail verdict; the evaluation sorts the lot into the systems the rule allows, and a standard trench is often not one of them. Table V of §285.91 marks a standard subsurface absorption system unsuitable where any of these is found: “Soil Class IV along the sidewall or within two feet below the bottom of the excavation”; Class Ia gravelly sand in the same zone; “A restrictive horizon intersects the sidewall or is within 24 inches below the bottom of the proposed excavation”; “Indications of seasonal groundwater or drainage mottles anywhere within 24 inches of the bottom of the proposed excavation”; slopes over 30 percent; floodplain or floodway; fill material. Section 285.31(b) then says what happens next: “If it is determined that a standard subsurface absorption system cannot be used, either a proprietary or a non-standard system may be used, provided all soil and site criteria for that system can be met.”
Table XIII lists what a Class IV clay lot may still have. Suitable: an evapotranspiration bed (lined or unlined, in the drier part of the state where evaporation beats rainfall), a pumped-effluent drainfield on ground sloping 2 percent or less, low-pressure dosing, drip irrigation, a mound where there is at least a foot of native soil and under 10 percent slope, and surface application, the aerobic spray system, with vegetation cover and disinfection. Unsuitable in clay: the standard absorptive drainfield, leaching chambers, gravel-less pipe and soil substitution. Low-pressure dosing, mounds, and surface application are nonstandard disposal systems, and §285.33(d) reserves planning materials to a professional engineer or sanitarian; drip is a proprietary system with its own approval rules.
That is why “failed the perc test” in Texas usually means “needs an aerobic system.” In Kaufman County, on Blackland clay, 443 of the 475 systems the county authorized in 2025 were surface-irrigation systems; the aerobic cost guide covers what that decision costs to build and to keep under contract.
Chapter 285 puts no expiry date on a site evaluation; the clock that runs is the permit’s, one year from issue. Section 285.3(d)(1): “An authorization to construct is valid for one calendar year from the date of its issuance.” No state rule and no county page found by this page sets an age limit on the evaluation report itself. What counties do instead is re-check the ground: Fort Bend County’s procedure says that at the pre-construction inspection “the inspector will verify that the design submitted to and reviewed by this office is accurate. Separation distances will be measured, water well locations will be verified, soils evaluated as needed.” Reusing an old report is a question for the county office, not a right the rule grants; the “one to two years” on the ranking pages is nobody’s rule.
Replacing a drainfield resets everything: under §285.2(61) it “is considered a repair and requires a permit for the entire OSSF system,” which means a new application and a current site evaluation, and TCEQ adds that “any work would require the system to meet the current standards.”
Order the evaluation before the earnest money goes hard, and read the county’s rules before assuming ten acres exempts you. Under §285.3(f)(2), a single-family dwelling on one tract of ten acres or more needs “No planning materials, permit, and inspections” if the system stays 100 feet inside the property lines, disposes on the property, and serves the only dwelling; no planning materials means no site evaluation is filed. Counties can remove that exemption, and several have: Hays requires a permit “regardless of the size of the lot or acreage,” Kerr has since August 7, 2026, and Kaufman and Medina say the same in their Orders. The 10-acre rule guide tracks which counties still honor it.
For a house that already has a system, the lender usually asks for something else. TCEQ: “Your mortgage company may require an evaluation of your OSSF before completing the sale. This type of evaluation is not required or regulated by TCEQ.” That is an inspection of the existing system, not a soil evaluation for a new one. USDA Rural Development calls it a “septic evaluation”; the Texas REALTORS on-site sewer facility form asks the seller for “planning materials,” the original permit and the final inspection. None uses the words “perc test.” The file should include the site evaluation report and the permit it supported, both on record with the county.
One Central Texas installer publishes $300 to $600 for the site evaluation alone and $500 to $1,200 with the engineering; no county charges for the evaluation itself. Evaluators do not publish rate cards, so get written quotes from two or three license holders.
No. A TCEQ-licensed site evaluator or a professional engineer must perform the evaluation, even if a homeowner installs the system personally. 30 TAC §30.244 exempts an owner-installer from the installer license. Then it requires that “The site evaluation must be performed by an individual who possesses either a current site evaluator or a professional engineer license.”
The evaluation cannot be failed; it decides which systems the lot may have. Clay, rock, or groundwater within two feet of the trench bottom rules out a standard drainfield under Table V, and Table XIII then allows an ET bed, a pumped-effluent field, low-pressure dosing, drip, a mound, or an aerobic spray system.
The rule sets no expiry on the evaluation; the authorization to construct it supports expires one calendar year after issue. Counties may re-check the soil at the pre-construction inspection, as Fort Bend County’s procedure does, so ask the permitting office before reusing an old report.
A TCEQ-licensed site evaluator or a Texas professional engineer. Some counties, Kaufman among them, accept only an engineer or a registered sanitarian holding the evaluator certification.
No; what a lender may ask for is an evaluation of the existing system’s condition, which TCEQ does not regulate. A new site evaluation is required only when a system is built, or when a tank or drainfield is replaced, which the rule treats as a repair needing a full permit.
Rule text was read on September 24, 2026 from 30 TAC §285.2, §285.3, §285.30, §285.31, §285.33 and §285.40 and 30 TAC §30.244 on Cornell LII, §285.91 Table V on Fastcase’s rendering of the 2021 Texas Administrative Code, and Table XIII on Justia’s copy of the 2024 attached graphic. TCEQ’s owner-advice page (last modified March 3, 2026) supplied the February 4, 1997 date and the mortgage-evaluation statement; TCEQ’s OSSF licensing page (last modified March 24, 2026) supplied the evaluator prerequisites. “Percolation” appears in none of the sections read; the whole chapter was not searched.
Texas cost figures are one installer’s published ranges, MAC Septic (March 4, 2026); Dillon Septic’s guide carried $600 to $1,000 (conventional) and $800 to $1,300 (aerobic) for evaluation and design when read on September 24, 2026, and its September 25 rewrite no longer prints them, so they were removed at the fact gate on September 27. Four Texas evaluator and design firms were checked, and none publish a price. National figures are Angi (September 15, 2026), HomeGuide (January 31, 2024), and Fixr (January 31, 2025), each recorded separately. County positions are quoted from Kaufman County’s 2022-approved Order, Medina County’s 2021 instruction sheet, Fort Bend County’s May 2023 procedures, and the Kerr and Hays County OSSF pages; lender and real-estate usage from USDA Rural Development’s October 2021 appraisal Q&A and Texas REALTORS form TXR-1407 (January 2026). The competitor page was read only to identify the claims corrected above.
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