Designing around hard site limits: shallow bedrock, high water table, steep slope, restrictive horizon, tight lot.

Standard absorptive drainfields need two feet of suitable soil from the excavation bottom to a restrictive horizon or groundwater, with excavations at least 18 inches deep, no deeper than three feet (five feet where annual precipitation is under 26 inches), at least 18 inches wide, no single run over 150 feet, and three feet of undisturbed soil between runs (30 TAC §285.33). Where rock is encountered, a standard system is allowed only if the rock horizon is at least six inches above the excavation bottom, suitable soil extends two feet below it, no groundwater is within two feet, runoff and subsurface flow along the rock are diverted, and the sizing formulas give no sidewall credit; alternatives for shallow rock, high water table or clay include lined ET (impervious liner where two feet of suitable soil to a restrictive horizon or groundwater is absent), mounds (min. 1.5 ft to a restrictive horizon, 2 ft to groundwater), LPD (2 ft to groundwater, 1 ft to a restrictive horizon), drip with secondary treatment (1 ft to groundwater, 6 in to a restrictive horizon), and soil substitution with a 24-inch Class Ib/II soil buffer, which is prohibited in Class IV (30 TAC §285.33; TCEQ, Choosing a Septic System). Slope limits: uniform slopes under 30% are suitable for standard absorption, excavations must parallel the contour, slopes under 2% require surface drainage controls, pumped effluent drainfields require slope ≤2.0%, mounds require slope <10%, and spray fields require ≤15% unless landscaped and terraced (30 TAC §285.31; TCEQ, Choosing a Septic System; TCEQ RG-472). Tight lots run into §285.4 minimum lot sizes of 1/2 acre (public water) and 1 acre (no public water) for subdivisions platted after the rule's effective date, with a narrow path for lots subdivided before January 1, 1988 (30 TAC §285.4).

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