Usually, home ads include only living space, which a garage is not.
When a house says it's such and such square feet does it usually include the garage in that number or is the garage separate?
There's really no rule.
If it's a separate garage, it's probably not included ... But if it's attached, I would recommend calling and asking. Or find the blueprints.
And check if it has a basement which is finished. Some count the square feet of a finished basement as part of the square feet. Or when you look at the home, take a tape measure and approximate width times length should equal 900. (might not be exact due to width of walls etc.)