A backflow preventer should be at least 12 inches off the ground, one foot, but no higher than four feet above the ground or floor. \
The backflow preventer needs some airflow and some empty space underneath it. This is in case the backflow preventer fills up and water pools up underneath it. \
However, height requirements for backflow preventers can vary based on city, county, and state laws. Check with your local office of plumbing permits to find out what the requirements are in your area. \