The garage would be a garage and a separate asset from the house. In the business section of TurboTax after you enter the income, you will be able to add assets. Add the garage as an asset, you will be asked for the cost, you will need to determine this based on tax records or possibly a prorated value based on the size of the garage vs the size of the overall house.
Then you will do the home office as a home office in the home office section. In this section you will again enter the square footage of the office versus the square footage of the entire home.
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