TBD SH 21 None is located in San Marcos, Texas, in Hays County. Hays County is one of the fastest-growing counties in the United States, driven by the explosive growth of San Marcos and Kyle-Buda corridor connecting Austin and San Antonio. The county blends Hill Country character in its western reaches, Wimberley, Dripping Springs, Driftwood, with rapidly urbanizing communities along the I-35 corridor.

The Blanco River and Barton Creek headwaters flow through Hays County, carving spring-fed swimming holes beloved by generations of Texans. Jacob's Well, a natural cylindrical karstic spring, and the swimming holes of Blue Hole Regional Park draw visitors year-round. The Edwards Plateau escarpment creates dramatic topographic relief in the western half of the county.

San Marcos had an estimated population of 68,920 as of 2023. The 78666 zip code has a median household income of $55,478, a median home value of $271,900, and a median age of 27. The average household size is 2.35 people.

Texas State University anchors San Marcos, supporting a significant education and healthcare sector. The I-35 corridor has attracted major distribution centers and manufacturing. The western Hill Country communities of Wimberley and Dripping Springs have become destination wine, dining, and retreat markets commanding premium real estate prices.