Reserve/replacement area — whether Texas design rules require setting aside a second disposal area.

Chapter 285 does not impose a general statewide reserve or replacement disposal area for a standard single-family OSSF — §285.30, §285.31, §285.4, §285.5 and §285.90 contain no reserve-area requirement, and TCEQ's permitting and system-selection pages state none (30 TAC §285.31; 30 TAC §285.4; TCEQ, Getting a Permit). There are targeted exceptions: under §285.81(g), when a disposal field is downsized because of a graywater or combined reuse system, a reserve area equivalent to the reduction must be shown available, must meet §285.91(10) setbacks, must carry no surface improvements, and must be disclosed in a recorded affidavit (TCEQ RG-472); and §285.4 requires planning materials for restaurants and buildings with food service to show land area adequate for doubling the land needed for the treatment units (30 TAC §285.4). Local orders can require one outright — Washington County requires proposals with more than one dwelling/RV per acre, or any commercial, institutional or multi-unit residential OSSF, to show "usable reserve area equal to all OSSF disposal areas on the subject property," with 25% of that reserve counted in the 440 GPD-per-acre cap via GPD/A = 1.25Q/Ac (Washington County OSSF Order 2025), while Hays County requires "reserve capacity" calculations in planning materials without defining a second disposal field (Hays County OSSF Regulations).

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