The best location for a thermostat in an attached garage is on the shared wall of the garage and the house. In a standalone garage, the best location is on the wall under the heater.
Garage thermostats work best when placed away from airflow and out of the direct path of the heater fan. An ideal location is a side wall roughly 4 to 5 feet above the floor. A good place to install a heater is in a corner, positioned at a 45-degree angle toward the garage door. Always check to see if permits are required before installing garage heaters.