A residential septic permit in Williamson County costs $510 for a conventional system, $610 for a non-conventional, non-aerobic design, and $710 for an aerobic treatment unit (ATU) on routine maintenance. Unlike most Texas counties, the state’s $10 TCEQ fee is carved out of that amount rather than added to it, so the number you see is the number you pay. Applications go in through MyGovernment Online, and the county says to expect up to 30 days for the initial design review.
The Williamson County Department of Infrastructure, County Engineer’s Office, runs the on-site sewage facility (OSSF) program for the entire county except areas inside the City of Austin.
Williamson County Department of Infrastructure — County Engineer’s Office, OSSF Program 3151 SE Inner Loop, Suite B, Georgetown, TX 78626 Phone: (512) 943-3330 · Fax: (512) 943-3335 Email: ossf@wilcotx.gov Payment hours (in person or by phone): weekdays 8:00 a.m.–4:30 p.m.
Not sure your property needs a permit? The county points you to its address search at gis.wilco.org/search: if the city shows as “Unincorporated” or lists an extraterritorial jurisdiction (ETJ), you likely need an OSSF. Call to confirm.
$510 to $910 depending on system type and whether the property is residential or commercial, with the $10 TCEQ state fee included in the permit price rather than charged separately.
From the county’s fee sheet, updated September 12, 2025:
| Permit type | Fee |
|---|---|
| Residential conventional | $510 |
| Residential non-conventional / non-aerobic | $610 |
| Residential aerobic treatment unit (routine maintenance required) | $710 |
| Commercial non-aerobic | $810 |
| Commercial aerobic (routine maintenance required) | $910 |
| Extra site visit / extra inspection | $100 |
| New or additional design review | $150 |
| License renewal or transfer (aerobic systems) | $60 |
| Subdivision review | $125 + $20 per lot |
| Inspection of an unlicensed OSSF | Cost of the applicable permit |
| Open records copies | $0.25 per page |
Two things are worth noting. These amounts have been remarkably stable: the county’s guide lists the same figures as “effective February 1, 2016,” and the 2025 fee sheet still carries them. And the aerobic tier is not just a higher fee, it starts a recurring obligation, because systems using surface application or drip irrigation need an ongoing service policy with a licensed maintenance provider and a license renewal with the Department of Infrastructure every two years at $60.
Pay by card online or by phone, or by mailed check or money order.
Six stages: online application, site and soil evaluation with open profile holes, design review of up to 30 days, written Authorization to Construct, phased inspections with green tags, then a License to Operate.
Apply online. MyGovernment Online is the only channel. Upload the warranty deed, property survey, and the OSSF design and soil report; if you are not the owner, add a signed permit authorization letter. Once staff deem it administratively correct they email an invoice, and after payment the portal emails a job-site identification tag with your permit number. Post that tag, but understand it authorizes no construction. Applications are valid one year from purchase.
Site and soil evaluation. A licensed Site Evaluator or Professional Engineer must assess topography, subsoil type and texture, soil depth, restrictive horizons, groundwater evidence, flood hazards, wells, recharge features, and setbacks. The part that surprises owners: you must excavate two profile holes, each 24 inches wide and five feet deep (or down to solid rock), at opposite ends of the proposed drain field, and they must stay open so a county inspector can verify the evaluation.
Design and plan review. A Professional Engineer or Registered Sanitarian designs the system to match the site evaluation. Exception: homeowners or licensed installers may prepare standard gravity-flow or evapotranspiration (ET) designs, but not if the site sits in the 100-year floodplain or the recharge zone. Aerobic plans must include the filed aerobic affidavit and service policy before review begins. The county states the initial review may take up to 30 days, handled in the order received.
Authorization to Construct. Written authorization must be granted before construction starts. It permits work by a licensed installer, or by the property owner on their own single-family dwelling with the department’s approval.
Construction and inspections. Build to the approved design; any change needs the designer’s written approval beforehand and an as-built drawing afterward. Inspections happen at critical phases, and the county expects installers to be ready during the morning or afternoon window they booked, equipment failure and weather excepted. Passing gets you a green tag; failing gets you a $100 re-inspection.
License to Operate. After final approval, the county issues the license, and your system records it.
A licensed Site Evaluator or PE for the soil work, a PE or Registered Sanitarian for the design, and a licensed installer for construction, with one narrow homeowner exception.
Anything aerobic, drip, or on constrained ground is professionals only. Verify licenses in TCEQ’s public registry before you sign.
Williamson County hosts every form below; download current copies rather than a contractor’s.
$510 for a residential conventional system, $610 non-conventional, $710 for an aerobic unit; commercial runs $810 to $910. The $10 TCEQ state fee is included in those amounts, not added on top.
The county publishes an initial design review time of up to 30 days, worked in the order received. Incomplete applications don’t enter the queue until staff deem them administratively correct, so a missing deed or survey costs you the wait twice.
Yes, on your own single-family home, with written authorization from the Department of Infrastructure, and only for standard gravity-flow or ET designs if you’re doing the design too. If your site is in the 100-year floodplain or the Edwards recharge zone, or the system is aerobic or drip, licensed professionals are required.
A service policy with a licensed maintenance provider, plus a license renewal with the county every two years at $60. Budget that alongside routine pumping costs, and see our aerobic maintenance contract guide for what a contract must contain.
Every fee and requirement here was read in a Williamson County document or in 30 TAC Chapter 285, each named below with the date we read it.
One caveat we would rather state than hide: the county’s current OSSF Guide link returns an empty file, so the process detail comes from the 12/01/2021 copy that still renders. We have emailed ossf@wilcotx.gov to confirm nothing material changed.
Back to the Williamson County septic hub for ground conditions, local rules, and licensed providers.
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