Septic on a small or subdivided lot — what makes a lot effectively unbuildable?

Existing small lots that miss the 1/2-acre or 1-acre minimum may still use an OSSF if they were subdivided before January 1, 1988 or had a site-specific sewage disposal plan approved between January 1, 1988 and the rule's effective date — but the system must still meet every other Chapter 285 requirement (30 TAC §285.4(b)(1)). A lot becomes effectively unbuildable when, after subtracting Table X setbacks, easements, ponds, and unsuitable soil, there is not enough usable area left for the disposal field; Collin County, for example, requires 1 usable acre (1.5 acres if there is a water well) for lots divided in 2008 or later, "usable" excluding most easements and ponds, and treats a lot voluntarily made smaller after the applicable date as ineligible for an OSSF (Collin County Development Services). Where no permitted OSSF can be installed on the dwelling lot, §285.4(b)(2) allows piping wastewater to a second property under common ownership with recorded easements and a deed affidavit stating the two properties cannot be sold separately (30 TAC §285.4(b)(2)).

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