You might want to keep in mind that you might have to relocate the energy source for the water heater also. Depending on the configuration, you could have to run a gas line or an electric line. Your circumstances could vary greatly, but these could prove to be pretty expensive also.
One more point to consider is that some areas have extensive requirements for water heater installations. Particularly in earthquake prone areas, your local code might have more restrictions than is typical. Be sure to check your local code.