Austin definitely has zoning rules. In fact, here's a link to our zoning commission.
http://austintexas.gov/cityclerk/boa...tings/54_1.htm
Rainey Street was upzoned a few years back. Rainey Street was a really unique situation where you had this little sliver of a single family residential neighborhood sandwiched between I-35, Town Lake and downtown. It was a vestige of the area's past when more single family homes were in the area before downtown expanded. Some of the houses had fallen into disrepair and those were the first to go as developers started redeveloping the area with high rises. The rest of the homes were remodeled and restored and became bars.
West Campus is another neighborhood that was upzoned, but it's a different situation because it's had considerable density for more than 50 years. The first high rise in West Campus was built in 1968. West Campus is predominantly student housing. It was upzoned after the existing density had started warranting taller buildings. Now lower density (but still large) multi-family buildings are being torn down to make way for taller buildings. In fact, the first high rise built in West Campus is slated to be demolished and replaced with a taller building. Its twin across the street is already doing site excavation.