It should be built in the winter when the ground is frozen, or at least partially frozen. The ground should also be compacted and well drained. Remember that a pool covers an area of 40 feet by 12 feet which can generate about 2 tons of heat per day, so it must have enough insulation to avoid heating up the surrounding area too much during warmer summer months. A pool cover will help keep your pool warm and insulate it from freezing temperatures over night. Be sure to build around trees because they hold water very tightly making them harder for ice dams to form after heavy snowfall occurs on top of them. If you cannot build right next to a tree then use fencing as baselines and completely surround all sides with material such as rock or brick that does not absorb water but still keeps out critters like raccoons and squirrels who may find their way into your yard through open holes in fence posts!