The height of a two-story house depends on a number of factors. The bare minimum height is likely 16 feet, although most two-story houses are between 20 and 25 feet high.
The reason for the variation in the height of two-story houses is that not all ceiling heights are the same. Some houses have 8-foot ceilings, others have 10-foot ceilings and some have 12-foot ceilings. Also, the fact that the second floor is the top story allows the ceilings on that floor to be higher than the ground floor. In addition, since most roofs aren't flat, the stop of a two-story house may be even higher than the total ceiling height on both floors.