If you’re going to build your own deck one of the first considerations after you’ve laid out your posts is how deep your footings should be set. This is determined by the frost line in your area and the map below (courtesy of Home Depot Outdoor Projects) should give you a general idea of how deep you need to go.
The bottom of your footings should sit 6 inches below the frost line so the posts won’t shift as the ground freezes and thaws. You should also check your local building codes and call the city and speak to someone in the engineering department just to be certain of how deep do dig.
If you find yourself in the cold zone, it might be best to hold off on your project until late spring. The weather will also be more tolerable to finish your project.