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San Marcos, Texas
Inside San Marcos' city limits, shrink-swell clay limits houses without basements on 56% of mapped land, the 4th-highest of 11 cities in Hays County. A shallow water table limits basements on under 1%.
That clay share is above Hays County's 14%. FEMA's flood data list 30 flood losses since 2016 in ZIP code 78666 overlapping the city. Of its 30,349 homes, 23% were built before 1980; the median dates from 2001.
Data as of October 13, 2026 · sources · in Hays County
expansive clay: shrink-swell limits slab homes
high water table: limits basements
flood losses since 2016 in 1 overlapping ZIP
of homes built before 1980
Foundation repair in San Marcos
Foundation repair in San Marcos: shrink-swell clay limits slab homes on 56% of San Marcos' mapped land, more than in 62% of Texas cities.
About 10,600 of San Marcos' 30,349 homes are detached houses.
| Who designs the repair | The ASCE Texas Section's repair guidelines say a Texas-licensed engineer should evaluate the foundation and design the repair (2022 guidelines) |
| Prices | Cost guide ranges compared |
Soil under San Marcos city limits
| Expansive clay (limits slab homes) | 55.6% |
| High water table (limits basements) | 0.2% |
| Septic fields very limited | 81.3% |
| Clay swell (LEP, by area) | 8.7% |
| High-swell land (LEP 6%+) | 56.0% |
| Mapped soil | 26,013 acres |
| Soil series | Share here | Series nationally: expansive clay |
|---|---|---|
| Houston Black | 15% | 91% |
| Branyon | 14% | 92% |
| Comfort | 11% | n/a |
| Heiden | 11% | 80% |
| Lewisville | 9% | n/a |
| Krum | 5% | n/a |
What the numbers mean here
Clay swell averages a linear extensibility of 8.7% by land area here, and 56% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 56% of the mapped land.
Clay shrinks as it dries: 100% of Hays County is in drought on the latest map, and 56% of San Marcos' mapped land has shrink-swell clay.
USDA rates 81% of the land very limited for septic absorption fields.
Drought in Hays County
100% of Hays County is in drought (D1 or worse) on the October 13, 2026 map. County detail.
Flood losses by ZIP code
FEMA flood records since 2016 in the ZIP codes that overlap San Marcos. ZIP areas reach past city limits.
| ZIP | Share of city land | Flood losses | Paid |
|---|---|---|---|
| 78666 | 91% | 30 | $779,955 |
Homes in San Marcos
| Housing units | 30,349 |
| Median year built | 2001 |
| Built before 1980 | 23% |
| Built 2000 or later | 52% |
| Owner-occupied | 31% |
| Median value, owner-occupied | $310,400 |
| Single-family detached | 35% |
Nearby cities
| Place | Distance | Expansive clay | High water table |
|---|---|---|---|
| Martindale | 6 mi | 70% | under 1% |
| Kyle | 9 mi | 51% | under 1% |
| Staples | 10 mi | 83% | under 1% |
| Uhland | 11 mi | 82% | under 1% |
| Mountain City | 12 mi | 17% | 0% |
| Wimberley | 12 mi | 18% | 0% |
| Niederwald | 13 mi | 84% | 1% |
| Buda | 15 mi | 61% | under 1% |
Sources: USDA SSURGO (August 5, 2026) · US Drought Monitor (October 13, 2026) · FEMA NFIP v3 (September 9, 2026) · Census ACS 2020-2024. Methods · Data status