Septic and OSSF on Hill Country acreage: what to know before you build

Why septic, not a building inspector, runs the show

Texas counties have limited authority to enforce a general building code in unincorporated areas, and the Hill Country counties generally do not. That surprises people who expect a building inspector. What you do answer to is the OSSF (septic) program, run by the county or its authorized agent under state rules in 30 TAC Chapter 285. If your project generates wastewater, a home, a shop with a bathroom, a barn with a wash rack, the septic permit is usually the approval that sets your schedule.

The 10-acre exemption, and its conditions

Under 30 TAC 285.3, a single-family dwelling on a tract of 10 acres or more can be exempt from the septic permit, planning, and inspection requirements. The conditions all have to be met:

• the tract is 10 acres or larger;
• it is a single-family dwelling and the only dwelling on the tract;
• every part of the system is at least 100 feet from all property lines;
• the effluent is disposed of on the same tract; and
• the system is not causing a nuisance or polluting groundwater.

That fifth condition is the one thin soil over fractured limestone most often fails, which is why the exemption is worth less on Hill Country ground than the acreage threshold suggests. The cost calculator prices the septic line the same way, assuming the permit applies.

Two cautions. First, the exemption removes the permit and inspection step only. The system still has to meet the construction and groundwater-protection standards in Chapter 285, so build it right regardless. Second, local authorized agents can be stricter. In Gillespie County, the county requires approved wastewater disposal in all areas regardless of acreage, so treat the 10-acre exemption as unavailable there. Always confirm with your county before relying on it.

Why most Hill Country sites need an aerobic unit

A conventional septic system relies on soil to absorb and treat effluent through a drain field. Much of the Hill Country is thin soil over fractured limestone, which does not provide that, and which can let untreated effluent reach groundwater. The common answer is an aerobic treatment unit that treats the wastewater before dispersing it by spray or drip. ATUs cost more up front and require a maintenance contract, but on karst ground they are frequently the only system that will pass. Budget for one unless a site evaluation says otherwise.

The Edwards Aquifer question

People often ask whether the Edwards Aquifer adds a layer of septic review. In the five western counties, Gillespie, Kerr, Kendall, Bandera, and Blanco, almost never: none is in the regulated Edwards Aquifer Protection Program. The single western exception is the southeast corner of Kendall County near Boerne, where the regulated recharge zone begins at Herff Falls on Cibolo Creek, and a Water Pollution Abatement Plan can apply. Hays and Comal counties are different. Both are regulated Edwards counties, and a septic system over the recharge zone there carries extra requirements under 30 TAC 285.40, including a minimum one-acre lot per single-family home and setbacks from recharge features. If you are building in Hays or Comal, or in that corner of Kendall, check the TCEQ Edwards map viewer for your parcel. Everywhere else, design to protect groundwater as a matter of course, but there is no Edwards review on your project.

How the counties differ

The state rules are the floor, but each county runs its own program. Gillespie requires a permitted system everywhere. Kerr County Environmental Health issues an Authorization to Construct you must have before installing. Kendall folds septic into a development-permit process. Bandera runs it through the County Engineer under a county OSSF Order. Blanco ties the septic permit to subdivision platting, so an unplatted lot can stall you. Hays runs septic through county Development Services with the Edwards recharge-zone rules layered on where they apply, and Comal runs it through the County Engineer's Office with the same Edwards overlay. Read your county page for specifics and the official contact.