Septic and floodplain steps in Gillespie County

On rural Gillespie County land, the septic system and the flood rules decide where the house can go. Gillespie County changed both in 2025. The County Engineer's office took over floodplain determinations on August 1, 2025, a new flood ordinance took effect July 28, 2025, and new septic rules took effect October 14, 2025.

This guide covers land outside Fredericksburg city limits. Inside city limits, the City reviews plans and issues permits, and the County's brochure sends questions about the City's extraterritorial jurisdiction (ETJ) to City Development Services.

Gillespie County's "Development Requirement Information" brochure, revised September 2025, covers development outside Fredericksburg city limits. It lists no county building permit. It does list:

• An on-site sewage facility (OSSF) permit from the Sanitation Department
• A floodplain development permit from the County Engineer
• A right-of-way permit for any driveway or construction entrance off a county road
• Subdivision review through the County Engineer, for land outside the City's ETJ
• Rural 9-1-1 addressing

The brochure also states that the county requires approved wastewater disposal in all areas of the county, regardless of acreage. A 40-acre tract needs a permitted septic system just as a two-acre lot does.

Step 1: The development permit determination

Start with the County Engineer's office. The county's Sanitation Office page says the Development Permit Determination Form and a Site Plan with Narrative go to the County Engineer's office before you submit the septic application. The septic application "will not be considered complete" until that office signs the determination form.

The form itself sets out what the review covers:

• No fee. The county charges no review fee for the determination.
• A site plan and narrative. You attach a plan and a written narrative describing all existing and proposed development on the property.
• Three checks. The floodplain administrator checks whether any part of the property lies in a special flood hazard area (SFHA), whether development falls within 100 feet of an SFHA, and whether impervious cover reaches 40% or more.
• Revisions void it. Any revision to the site plan voids the determination.

Settle the house location, the shop, the driveway, and the septic field before you file. Moving the house after the determination means filing again.

Step 2: Check the flood layers yourself

You can check the maps yourself first. The county brochure points owners to two federal tools: FEMA's Flood Map Service Center and the USGS estimated base flood elevation viewer.

The FEMA map alone is not enough here. Gillespie County's Flood Damage Prevention Ordinance adopts FEMA's Flood Insurance Study and flood maps dated October 19, 2001. It also adopts FEMA's Base Level Engineering 1% flood extents as Zone A special flood hazard areas, and the Base Level Engineering 10% extents as floodway. A creek that shows no floodplain on the 2001 map can still carry regulated Zone A along its banks under the newer layer.

Check the driveway route as well as the house site. A low-water crossing on the only way in can strand you after heavy rain, whatever the map says about the house.

Step 3: Floodplain rules if you build near water

The ordinance names the County Engineer, or a designee, as the Floodplain Administrator. A floodplain development permit is required for development in a special flood hazard area. The elevation rules that follow shape the foundation:

• In the SFHA. A new or substantially improved residential structure must have its lowest floor, including any basement, elevated at least one foot above the base flood elevation (BFE).
• In Zone X. A residential structure must submit a site plan that shows the floodplain boundaries.
• Within 100 feet of an SFHA. A structure must submit an Elevation Certificate confirming that the lowest adjacent grade sits above the BFE.

If your building site sits near a creek, ask early who will prepare the Elevation Certificate and what it costs, and set the pad height with the BFE in hand.

Step 4: The septic site evaluation

TCEQ's permit steps put a preconstruction site evaluation ahead of the application. A site evaluator who holds a TCEQ Site Evaluator license, or a Texas professional engineer, does the evaluation.

The evaluation looks at the soil where the drain field would go. In NRCS soil survey data, 67% of mapped Gillespie County acreage falls in soil units that include bedrock within about 20 inches of the surface. Shallow rock can limit which system the site supports. You can look up the mapped soils for a parcel in Web Soil Survey before the evaluator visits, but the evaluation decides.

Step 5: The OSSF permit and installation

With the signed determination and the site evaluation in hand, you submit the septic application to the county Sanitation Office. Gillespie County's current rules are the "Order Adopting Rules of Gillespie County, Texas for On-Site Sewage Facilities," effective October 14, 2025, and new OSSF fees took effect on that date as well. Ask the Sanitation Office for the current fee schedule.

State rules set a few more points:

• Local rules can be stricter. TCEQ says local requirements can be more stringent than state minimums, so the county order controls where it goes further.
• Thirty days. The permitting authority must approve or deny an application within 30 days of receiving it.
• Approval comes first. The county brochure says the application must be approved before construction on the OSSF starts.
• Who installs. If local rules allow it, a homeowner may install an OSSF for a single-family dwelling they own, on property they own, where no part of the property will be developed for sale or lease. Anyone else must hold an installer license of the correct level. An Installer I may install standard systems, and an Installer II may install all types.

Putting the site plan together

Before you file the determination and the septic application, leave room on the site plan for the drain field the evaluation calls for, plus a reserve area if the evaluator recommends one.

The guide to wells, groundwater districts, and power covers the well side of the site plan. For the full planning sequence, see the Texas Hill Country barndominium planning guide.