The Extraterritorial Jurisdiction (ETJ) is the unincorporated land within three and one half (3.5) miles of San Marcos’s boundary that is not within the city limits or ETJ of another city. It is the territory where San Marcos alone is authorized to annex land.

In accordance to the Texas Local Government Code Title 2 Subtitle C Chapter 42, interlocal agreements between the City of San Marcos and the City of Kyle created (Res 1987-072) and updated (Res 2013-034) an ETJ Agreement line where ETJ overlap occurred. Additional agreement lines with the City of New Braunfels (Res 1987-76R) and Uhland (2013-076) are also reflected in this feature.

In 2016, a map error was discovered in the northwest area of the ETJ. This error has perpetuated in the data since 2007, when the city declared a population of 50,000 which extended the ETJ to 3.5 miles (Resolution 2007-146-R). After discussion with the City of Kyle the adjustment was made to correct this error in February 2017. The area consisted of 103 parcels northwest of Owl Hollow Rd and northeast of Hilliard Road.