Building a barndominium or barn in Blanco County: permitting and site work

Blanco County does not enforce a general building code in the unincorporated county. It does regulate subdivision platting and floodplain, and it ties the septic permit to the platting status of your lot. The City of Blanco and the City of Johnson City handle building permits inside their limits. Out in the county, the septic permit, and whether your lot is in a properly platted subdivision, are the steps that govern.

The on-site sewage facility permit, the septic permit, is usually the binding approval for a rural Hill Country build. In Blanco County it runs through the Blanco County OSSF Inspector. Mailing address in Johnson City. Confirm the current phone, email, application, and fee on the county OSSF application and checklist before you file.

Local detail: Blanco County has a rule worth knowing early: no septic permit will be issued for a lot in a subdivision unless a final plat for that subdivision has been approved and filed of record. If you are buying a lot in an older or informal subdivision, confirm the platting status before you count on being able to permit a septic system, because an unplatted lot can stall the whole project.

Visit the official Blanco County page to confirm current forms, fees, and contacts.

Do not count on the statewide 10-acre exemption in Blanco County until the county confirms it for your tract. Blanco County is the septic authority for the whole county outside Johnson City and Round Mountain, and its Development Rules (revised January 23, 2024) say a permit must be obtained from the county Inspector's Office before construction of a system begins. Those rules point to separate county OSSF rules that are not posted online, and nothing the county does post addresses the exemption, so the county decides. Its OSSF application checklist (updated October 2025) adds that any land platted after 1998 needs at least five acres for a septic system and a well, that the county requires at least a 1,000-gallon tank, and that a permit is good for one year; the single-family residence permit fee is $300, or $375 where a licensed professional design is required. Call the Blanco County septic inspector at 830-868-2117 before you rely on any exemption. Read October 16, 2026.

Blanco County is not in TCEQ's regulated Edwards Aquifer Protection Program, so a building project here does not require an Edwards Water Pollution Abatement Plan. The county is upstream of the Edwards and drains toward it through the Blanco and Pedernales rivers, and most of it is underlain by the Trinity aquifer. Groundwater protection still shapes septic design, but the formal Edwards recharge review does not apply.

The Hill Country sits in the roughly 105 to 115 mph design wind speed band (3-second gust, Risk Category II) under ASCE 7-16, the standard most Texas building codes reference. Most parcels fall between mapped contours, so a designer pulls the exact value for your address from the ASCE Hazard Tool. The local twist is the topographic factor, Kzt: an exposed ridge, hilltop, or escarpment raises the effective wind pressure on a building, sometimes by half again versus flat open ground. A metal or post-frame building on a Hill Country ridge should be engineered for that, which is one reason a sealed design from the builder matters here.

Most of Blanco County draws on the Trinity aquifer, and the Blanco and Pedernales rivers run through it. If your plan depends on a new well, confirm registration with the applicable groundwater district and budget for a limestone drill alongside your septic system.

A 1-d-1 open-space valuation from the Blanco Central Appraisal District lowers the tax valuation on qualifying land, not on buildings. A barn, shop, or barndominium is appraised separately at market value. There is no county square-footage threshold that makes a building permit-free, and ag valuation does not remove the septic permit or the platting requirement. For a structure tied to a qualifying ag use, ask the appraisal district directly.