I purchased home in 1996 and most recently refinanced in March 2021. That same month I began renting out a room. What year price is cost basis?

[ColeenD3]

Your cost basis is what you paid for the property. Your mortgages have nothing to do with it. Prorate the business portion and personal portions according to square footage.

[Carl]

Your cost basis has nothing to do with any mortgage.

When converting a personal use property such as your primary residence or 2nd home to residential rental real estate, the cost basis is what you paid for the property when you originally purchased it, plus the cost of any property improvements you paid for since you owned it.