To reduce the fire and explosion hazard of gas or electric hot water tanks located in garages and other hazardous areas where flammable liquids are stored, the 1997 International Mechanical Code Commentary's Section 304.2 requires that appliances that have an ignition sources shall be elevated to a minimum of 18 inches above the floor in hazardous locations and private garages. This is due to flammable and combustible liquids give off vapors which are denser than air and tend to collect near the floor. This section effectively prohibits the installation of most heaters, furnaces, dryers, and water heaters directly on the floor of residential garages, and could be extended to include basements if hazardous liquids are stored there.

This section is used by appraisers and inspectors, and if it is a requirement to raise a hot water tank as part of a contract of sale, the City of Tulsa requires a plumbing permit and an inspection. However, the City is mainly concerned with proper and safe installation and there is no requirement that everything else has to be brought up to today's code. According to Stephen Scott, Assistant Chief, City of Tulsa Plumbing Inspection, sellers and even buyers should confirm that such work be done only by a licensed plumber with a permit ($33). Failure to do so can result in a $100 fine to whichever is the current homeowner, and who will then have to apply for the permit and inspection. For permit requirements in outlying communities, it is recommended that you contact that town's Plumbing Inspector.

source - Greater Tulsa Association of Realtors Newsletter