An extension ladder reaches roughly 3 feet less than its overall height due to the overlapping of the base and flying portions. As a result, the needed ladder size for a two-story house is 21+3=24 ft., with a maximum reach of 21 feet. Ladders are usually sold in standard sizes, such as 24 feet, so this calculation is simple! However, safety factors should be taken into account when using extension ladders. The National Institute for Standards and Technology recommends that an assistant be present when anyone is standing on an extension ladder. This person can call for help if necessary. It also recommends that an electric cable or chain not be used as a stairway because it's easy to trip over it. Finally, extendable ladders are available in various lengths and grades of strength. If you need a longer ladder, then an extension ladder is the way to go. Otherwise, you may not need as long of a ladder after all!