Retaining walls have multiple applications: smaller retaining walls may sculpt a sloped yard into a terraced garden; larger walls may be asked to support a slope above which a tract of homes is built. According to the Uniform Building Code, all retaining walls over four-feet-tall (from the slope beneath it) built in the United States require professional engineering and a permit, but states, counties and cities may issue their own building codes. Below are examples of the maximum height that retaining walls may be built without a permit in some larger cities. Akron, OH. 3 ft. Albany, NY 4 ft. Boston, MA 4 ft. Denver, CO. 3 ft. Fort Worth, TX 4 ft. Las Vegas, NV All retaining walls require a permit Long Beach, CA 2ft. Portland, OR 4 ft. Savannah, GA 3 ft. St. Louis, MO 2 ft.