You use the lower of (1) your Adjusted Basis (usually original purchase price plus cost of improvements) or (2) the Fair Market Value on the date of conversion to a rental property.
If I understand your situation, that means you would probably use your Adjusted Basis for depreciation.
If you do not know the cost of the land when you purchased it, then your tax bill values could be helpful to determine the ratio of land versus the entire cost. For example, if your tax bill shows a total value of $100,000 and $20,000 of that is for land, that would be 20% for land. So you would take your actual purchase price (or Fair Market Value, if that is lower), and estimate 20% was for land. Does that make sense?
