When a Professional Engineer's seal is required on the design instead of a site evaluator's or installer's plan.

An owner or installer may prepare planning materials only for proposals that are not covered by §285.5(a)(2) or (a)(3); a professional engineer or professional sanitarian must prepare, seal, date and sign materials for non-standard treatment or disposal proposals, OSSFs serving manufactured housing communities, RV parks and rented/leased multi-unit residential developments, all §285.4(c) subdivision and development plans, multiple treatment/disposal systems on large tracts, and all new construction in the Edwards Aquifer Recharge Zone (30 TAC §285.5). A professional engineer only (sealed, dated, signed) is required for all non-standard treatment systems requiring secondary treatment, for verification that precast concrete septic tanks meet §285.32(b)(1)(E)(i), and for designs demonstrating compliance with the regulated-floodway requirements of §285.31(c)(2) — where the PE must show the system will not raise flood height, that components are buried without added fill, and that alarms, junction boxes and compressors sit above the 100-year flood elevation (30 TAC §285.5; 30 TAC §285.31). TCEQ's system-by-system table shows drip irrigation, LPD, mounds, soil substitution, surface application and secondary treatment must be designed by a PE or professional sanitarian, while non-standard treatment requiring secondary treatment must be designed by a PE (TCEQ, Choosing a Septic System); counties may narrow this further — Williamson County directs owners to a PE or registered sanitarian and allows homeowner/installer-prepared plans only for standard gravity and ET systems outside the 100-year floodplain and recharge zone (Williamson County OSSF Guide).

Reviewed August 2026. Links go to the sources this answer was checked against.