As used in this chapter, the following terms shall have the meanings indicated:
A structure designed, built or occupied as a shelter or a roofed enclosure for persons, animals or property and used for residential, business, mercantile, storage, commercial, industrial, institutional, assembly, educational or recreational purposes. A structure having dimensions of less than 20 feet by 20 feet at the base or having a height less than 15 feet shall not fall within the meaning of this definition.