Usually yes for a gas unit: its ignition source must sit at least 18 inches above a garage floor, because flammable vapors pool low. Modern FVIR (flammable-vapor-ignition-resistant) heaters are exempt from the elevation. Either way, protect the heater from vehicle impact.
Why the 18-inch rule exists
A garage is the one room in a house where people routinely store gasoline, mowers, paint thinner, and other flammable liquids. Those liquids give off vapors that are heavier than air. Instead of rising and dispersing, the vapors settle and form a thin layer right along the floor. A spark or an open flame at that level can ignite the layer and cause a flash fire or an explosion.
A gas water heater has a live ignition source. It has a standing pilot or an electronic igniter, and the burner flares up every time the tank calls for heat. Set that flame on the floor next to a leaking gas can and you have a hazard. Raise it 18 inches and the flame sits above the danger zone where vapors collect.
The same logic explains why the rule is tied to the height of the flame and not the height of the tank. What matters is where the spark or burner lives, since that is the point that can light a vapor. On an older heater, the whole unit goes up on a stand for that reason: the burner is at the very bottom of the tank, so the only way to lift the flame is to lift the tank.
The International Residential Code states the requirement plainly. IRC Section M1307.3 reads that appliances "having an ignition source shall be elevated such that the source of ignition is not less than 18 inches above the floor in hazardous locations and public garages." The International Fuel Gas Code carries the same language in Section 305.3. The 18-inch figure is not a guess; it is the height the model codes set as the buffer above a pooled vapor layer.
This applies to gas water heaters and to any appliance with a flame or igniter, such as a gas furnace placed in the garage. It is one of the first things a permit inspector checks on a garage gas appliance.
The FVIR exception for modern water heaters
The biggest change to this rule came from the water heaters themselves. Since the mid-2000s, residential gas tank heaters have been built with FVIR technology, which stands for flammable-vapor-ignition-resistant. An FVIR unit pulls combustion air through a flame-arrestor screen at the base and seals the burner chamber. If a flammable vapor reaches the unit, the design is meant to keep any ignition contained inside the chamber rather than letting it flash back out to the room.
Because an FVIR heater is engineered to handle a floor-level vapor on its own, the model codes and most manufacturers treat it as exempt from the 18-inch elevation. IRC M1307.3 includes an exception for appliances "listed as flammable vapor ignition resistant." Manufacturers like A.O. Smith and Bradford White (Bradford White brands its version the Defender Safety System) build this protection into their standard gas tank heaters, so a new compliant unit can sit on the garage slab.
A few cautions go with that exception. The FVIR system only works when the air-intake screen at the base stays clean. Lint, dust, and pet hair can clog that screen, starve the burner, and trip the unit, so it needs occasional checking. Local rules can also still require elevation, and an older pre-FVIR heater on a stand should stay on its stand. When in doubt, follow the unit's installation manual and your local inspector, since the manual reflects how that specific model is listed.
One practical note for replacements: not every gas tank heater you can buy is FVIR-listed, and a used or salvaged unit may predate the technology. If you cannot confirm a heater is FVIR-listed from its label or manual, treat it as a unit that needs the 18-inch stand. Building the stand to the maker's spec is cheap, and it keeps the install legal if the listing is ever in question.
Protecting the heater from vehicle impact
Elevation handles the vapor risk. A separate rule handles the other obvious garage hazard: a moving car. A water heater that sits in the path of a vehicle has to be guarded against impact. IRC M1307.3.1 requires that "appliances located in a garage... shall be protected from impact by automobiles."
There are two accepted ways to meet it. The first is location: put the heater where a car cannot reach it, such as in a recessed alcove, a closet, or against a wall behind where the vehicle parks. The second is a physical barrier, usually a steel bollard (a pipe post set in concrete) or a sturdy guardrail in front of the tank. Either approach keeps a bumper from crushing the gas line, the controls, or the tank shell.
This rule is independent of the 18-inch question. An FVIR heater that skips the elevation still needs impact protection if it sits where a car could hit it. The two requirements solve two different problems.
Electric heaters, and why this is common in Phoenix
An electric water heater has no pilot, no burner, and no open flame, so the 18-inch ignition-elevation rule does not apply to it in the same way. That said, an electric unit is not free of garage rules. Its vehicle-impact protection requirement is the same, and local codes can add their own conditions on wiring, drain pans, and placement. Switching from gas to electric to dodge a stand can make sense, but it does not remove every garage requirement.
Garage water heaters are typical in Phoenix. Homes here are usually slab-on-grade with no basement, so the garage is the natural spot for the tank along with the furnace and the laundry hookups. That puts a lot of local water heaters in exactly the room where the 18-inch rule and the impact rule were written to matter. Phoenix adopts the current Uniform Plumbing Code along with the IRC, IMC, and IFGC families, so the fuel-gas elevation and impact provisions described here are the ones enforced on a local permit.
If you are replacing a garage water heater, this is a good moment to confirm the install meets code. A new gas unit should be FVIR-listed or set on a proper stand, the impact protection should be in place, and the work generally needs a permit. For the related clearance and access rules, see water-heater-clearance-and-access-code. For combustion and venting safety, see can-a-water-heater-cause-carbon-monoxide. And before you start a swap, see permit-to-replace-water-heater-phoenix.