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.

Outside every city, the Blanco County Development Rules and Regulations (revised January 23, 2024) call for a plat whenever an owner splits a tract into two or more pieces, and Section 101.110 counts a division made by a metes and bounds deed or a contract for deed the same as one drawn on a map. The reason a home buyer cares sits in Section 101.190: the county issues no septic permit on a lot in a subdivision whose plat was never approved and recorded.

Section 104 lists the divisions that skip the plat, each with its own test. Every new lot over 10 acres with at least 60 feet of fee simple frontage along a state highway, a county road or a road built to county specifications. A single split into two tracts larger than six acres, each with 250 feet of that frontage. Four or fewer parts, each going to someone related to the owner within the third degree. Land that stays in agricultural, ranch, wildlife management or timber use. A split a lender requires on a tract of at least 12 acres with 250 feet of frontage, into no more than two parts with the smaller one at least five acres. And a strip bought by a neighbor to join to land the neighbor already owns.

None of them is automatic. The owner files a notarized Affidavit of Land Location and Exception with the commissioner for the precinct, with a plat by a registered surveyor or licensed engineer that shows the planned well and septic spots and no lot narrower than 60 feet at any point. Section 104.100 says the exception is denied unless the commissioner signs an Order of Acceptance, and an affidavit recorded without that order exempts nothing. Where a plat is needed, new subdivision applications are dropped off by appointment on the dates in the county's 2026 calendar, twice a month, each window paired with a Commissioners' Court date about four weeks later.

Changing a recorded plat, to combine two lots or move a line, runs through the County Judge's office under the county's replat procedures (updated January 23, 2024). The county publishes notice in the newspaper three times and mails it where Local Government Code 232.009 requires, which normally takes four to six weeks before the request reaches the court. The application fee is $300, newspaper notice is the actual cost plus $75, mailed notice is the actual cost plus $175, and the County Clerk's filing fee comes on top. A revision that touches a utility easement needs the utility's written release before the court approves or files it.

Section 300.240 of the Development Rules sets the smallest new lot by where the water comes from and where the wastewater goes. A home site drawing on a private well and draining to its own septic tank needs five acres outside the floodplain and 250 feet on the road, and a subdivision may hold no more lots than its total acres divided by six. A flag lot needs its five acres without counting the strip that reaches the road. On a TCEQ-approved public water system drawing groundwater, with septic on each lot, the floor drops to three acres and 150 feet of frontage at one lot per four acres, and the roads and drainage in that subdivision are kept up by someone other than the county. With public water and a wastewater treatment system, frontage is 100 feet, at one lot per four acres on groundwater or one per three on surface water or rainwater catchment.

Setbacks travel with every building site on a road the county regulates: 50 feet from the front line, and at least 10 feet off the side and back lines of a home site, which the rules give as fire protection spacing (the lot size and setback rules, Section 300.170). Lots in a subdivision recorded before these rules keep their old lot size, frontage and setback limits under Section 101.150, until someone divides or combines them. A lot on its own well also answers to the Blanco-Pedernales Groundwater Conservation District, whose registration and spacing rules the Johnson City page walks through.

Section 101.170 settles the edge of town. Land wholly inside a city's limits or its extraterritorial jurisdiction is under that city by interlocal agreement with the county. Land split between a city's jurisdiction and the county follows each one on its own side unless the two signed an agreement giving the city the whole of it, and land near Round Mountain answers to whichever of the town's rules and the county's is stricter.

A new private, public access or commercial driveway onto a county road needs a permit from the County Commissioner's office first, and Section 316.200 leaves the owner responsible for upkeep of the part inside the road right-of-way. Where the drive crosses a road ditch, the owner's engineer sizes the culvert to carry at least a 10-year flow at the low point of the lot's ditch, with a 15-inch minimum for a private drive and 18 inches for a commercial or public access drive, in corrugated galvanized metal or reinforced concrete pipe with concrete headwalls (Section 316.410).

A private drive is 10 to 24 feet wide where it meets the right-of-way, and a commercial drive 30 to 45 feet, with at least a five-foot radius on each side and an angle to the road between 60 and 90 degrees. Rock, concrete, metal or wood facings, fences and gates stay out of the right-of-way, and Section 316.550 sets every entrance and gate at least 40 feet back from the county right-of-way. That setback shapes where a ranch gate, a stone entry or a cattle guard can go before the barn is even sited.

The driveway permit also brings the address. If the location has none, the county assigns one with the permit, and Section 316.310 says the county GIS office needs a copy of the access permit before it issues a 911 address. The number goes up within 10 feet of the driveway entrance, facing the road, in reflective block letters at least four inches tall, so a fire truck or an ambulance can find a new house on a long caliche drive.