How do you you calculate how many square feet you have removed from a house of 2000 square feet if you remove a space 5 feet wide and 800 feet long?

An area of 5 feet by 800 feet is equal to 5 * 800 = 4,000 square feet.
How you can remove 4,000 square feet from an area of 2,000 square feet is a separate and nonsensical question however.