You should list the carriage house as purchased in 2018. Your basis is the cost of building it. The land does not get depreciated, so its value it unimportant to the depreciation question.
So, total basis for the carriage house is $241,000, and the land value is $61,000. Your basis for depreciating the carriage house is $180,000.
You should think of these as two separate properties. One is your home and one is a rental. Don't say that you used the property personally before it was a rental, because the rental did not exist before 2018.
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