If you are in a cold climate the best storage garage is one with radiant heat pipes in the floor. A heated floor will keep moisture away and keep your vehicles at a more constant temperature. It's more costly to build in radiant heat, but it actually costs less to heat a space this way than through a conventional heater.
Unheated storage is more common. Many times unheated garage storage means the car will be exposed to daily temperature cycles as the sun warms the building and it cools at night. This is harder on woodwork (if your car has woodwork) but it's not a big deal otherwise provided the air is dry and the car has antifreeze protection.
Barns are the worst places to store cars because they tend to be full of rodents and they often have dirt floors that wick moisture up into your chassis, rusting frames and brakes and anything else.
John Elder Robison is the general manager of J E Robison Service Company, independent restoration and Bosch Authorized Car Service specialists in Springfield, Massachusetts. John is a longtime technical consultant to the Land Rover, Porsche, and Rolls Royce Owner's Clubs, and he’s owned and restored many of these fine vehicles. Find him online at www.robisonservice.com or in the real world at 413-785-1665