Although a garage is attached to the home, it is not considered part of the home's square footage. That is because only livable space is considered in the square footage calculation.
Calculating the square footage of a home is not as easy as it sounds. Neither real estate agents nor homeowners should attempt the calculation (at least not if you want a reliable figure). Rarely are houses perfectly square, which is one reason for the difficulty. Appraisers map out the house on a piece of graph paper, calculate all the edges, come up with "mini-areas" for each rectangle - then add them all together.
Plus, there are other intricate rules. If there has been an addition to the house and the owner did not receive a building permit, then that section of the house may not be allowable as part of the square footage. The same with attic and basement conversions, lofts, and so on.
It is best to rely on a licensed appraiser to recalculate the square footage of a house.
When a home's square footage is advertised, the figure usually comes from previous sales, perhaps as far back as the builder. Homeowners and real estate agents don't usually recalculate the square footage. Like we said, it is very very difficult to calculate the square footage of a home.