For example, let's say your mother bought her house with all of the land for $100,000. Let's say the land valuation is 20% of the total purchase (as Carl said, the property tax assessor often will indicate the ratio).
So the total purchase price of all of the land was $20,000. Then if your mother gave you exactly half of the land, your 'cost' would be $10,000 (assuming both lots of land were of similar value).
Yes, you then add the cost of building the house to the cost of the land. That is your "Cost Basis". Once you have that, then you can determine which is lower, your Cost Basis or the Fair Market Value when you started to rent it.
