Fair market value is whatever is on the tax rolls since you haven't had it appraised.
That needs clarification. There is a vast difference between tax value and fair market value. While all tax rolls will show the tax value, they don't all show the fair market value. Typically based on what I've seen, the tax value is about 30% lower than the fair market value.
In my county, they just recently started showing FMV on the tax bills a few years after the crash of 2008/9. Before, all you got/saw was the assessed tax value.
Now-a-days when converting a property from personal use to rental use, it is not that common for the value on the date of conversion to be lower than the original acquisition price/value. However, depending on the locale it's perfectly possible for the assessed tax value to be lower. That's why the IRS does not condone the use of tax values for depreciation. (I've not seen anything that forbids it. Best I've seen is where it gives me the impression it can be used only when all other methods of determining FMV have been exhausted.)
