Designed for indoor use, treadmills may suffer if stored outside for an extended period, given their delicate motors, belts and speed/incline functions. Examine whether your treadmill manufacturer voids the warranty if the machine is not kept in a climate-controlled environment. That means the outdoors, porches, a garage, and even bathrooms are off-limits for the machine.
You can weigh other options if you want to be outside. You can roll your treadmill outdoors during your workout and then return it to your home for storage. Other options that may fit your workout goals include a rebounder, or mini-trampoline. You can walk and run outdoors in the fresh air, and then easily transport it inside. Small exercise machines that only have foot pedals, such as mini-steppers, are also easily moved outdoors and back in.