You can use any reasonable method for dividing the expense. While the most common methods for dividing an expense are based on the number of rooms in your home or based on the square footage of your home, you could use another way.
In some areas, non-airconditioned areas are given half the square footage when assessing property values. It would be sensible to assume that the Fair Market Rent of a garage would much less than a room in the house. In this case, in might be reasonable to use a Fair Market Rental method. For example, if you get $500 a month for a room and $250 a month for the garage, you might use that in calculating your allocation.
