Decks are NOT included in square footage.
The question of how home appraisers calculate a home's square footage is often more of opinion than of exact science. For example, one appraiser may consider portion of a finished basement in square footage where another will just adjust the home's value slightly upward based on similar homes in the area without finished basements. Square footage is based on actual ‘living space' of which decks do not qualify. Decks may be similarly given a credit adjustment rather than considered in square footage.
The basic principle of measuring square footage is actually very simple. From the outside of the house the structure is measured by length and width then multiplied by how many stories of heated living space are used year-round. An outdoor porch that is not heated or cooled would not be included in the square footage etc.