The square footage of a house is found by multiplying length times width, so a house that is 30 feet by 40 feet measures 1,200 square feet, assuming the house has only one story. The square footage of additional stories should be added as part of the total square footage.
The American National Standards Institute suggests using external measurements when calculating the square footage of a house. Any rooms that are not finished or heated like the rest of the house, such as an attic or a basement, should not be included in the total. A garage should also not be included as part of a house's square footage, though it is counted when calculating the footprint of a house.