for your original 50% you should have been taking 100% of yearly depreciation using only your cost not 50% on 100% of cost (my opinion)- i'll skip on whether you should have been filing partnership returns for the years your friend had his interest.
you now create a second asset for what you paid your friend and start depreciation with the purchase date in 2019
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for the 2011 remodeling you should have only included 50% of the cost and taken 100% of the yearly depreciation
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when you bought your friends interest you also bought the improvements so there is no separate asset for this
