Quick answer: A perc (percolation) test measures how fast water drains through your soil, and it decides what kind of septic system your land can legally have. In Texas the modern version is a soil evaluation performed by a licensed site evaluator, typically costing $300–$700. On Ellis County’s Blackland clay, most lots fail for conventional drain fields and are approved for aerobic systems instead. You cannot get a septic permit without one.
What a perc test actually measures
The classic perc test digs holes in the proposed drain field area, soaks them, then times how fast the water level drops, expressed in minutes per inch. Fast-draining sandy soil passes easily; slow clay fails. Texas has largely replaced timed hole tests with a soil evaluation: a licensed site evaluator examines soil texture, structure, restrictive layers, and seasonal water table in test pits or borings, then classifies the soil under 30 TAC Chapter 285. Same purpose, more complete picture. Everyone still calls it a perc test.
Perc test cost in Texas (2026)
Service
Typical cost
Soil evaluation, standard residential lot
$300–$700
Soil evaluation + full system design package
$700–$1,500
Large acreage or multiple test sites
$800–$2,000
What happens when soil fails for a conventional field
Failing is normal here. Most of Midlothian, Waxahachie, and the surrounding county sits on clay that percs far too slowly for gravity drain fields. A failed conventional evaluation does not mean you cannot build; it means the design moves to an aerobic treatment unit with spray distribution, occasionally a drip or mound design on difficult sites. The evaluation result feeds directly into the permit application, which is why the evaluator and the designer are often the same license.
When you need one
Building on raw land: before you close if possible. A soil evaluation is cheap insurance against buying a lot that needs a $20,000 engineered system.
Replacing a failed system: if no evaluation is on file, or the field is moving to new ground, the county wants current soil data.
Adding bedrooms: expansions that increase design flow can trigger re-evaluation of capacity.
Buying rural property: pair the soil picture with a septic inspection during your option period. The inspection tells you the system’s condition; the permit file and soil data tell you what the land can support if it ever needs replacing.
How the process runs in Ellis County
1. Licensed site evaluator visits and digs test pits in the proposed disposal area.
3. Report and design go to the permitting authority for your parcel.
4. Permit issued, install proceeds with county inspections at the required stages.
Timeline: the evaluation itself is a single site visit; permitting typically runs 2–6 weeks. Rainy season can slow pit work in clay.
Frequently asked questions
How much does a perc test cost?
$300 to $700 for a standard residential soil evaluation in North Texas, or $700 to $1,500 bundled with the septic system design.
How long does a perc test take?
The site work is usually done in one visit of a few hours. The written evaluation and design typically follow within days; county permitting adds 2 to 6 weeks.
What if my land fails the perc test?
In Ellis County a failed conventional evaluation is the normal outcome and simply routes the design to an aerobic system with spray, drip, or another advanced method. Very few lots are truly unbuildable for septic.
Who can do a perc test in Texas?
A TCEQ-licensed site evaluator. Many are also licensed designers or installers, which streamlines the permit package.