Calculating cost basis for rental

[Swati Patel]

Hi all, appreciate your time in reading my post and taking the time to respond. My question is as follows.

I bought my rental property in 2016 for $300K and the property tax letter accessed land value as $120K and structural value is $180K. Fast forward to 2020, the value of the rental has appreciated to $330K and property tax letter has now assessed land value as $130k and structural value as $200K. 

Which number of structural value should I use for cost basis? the value from 2016 when I purchased the property or the recent value?

If I use the recent value, then every year the cost basis will change because the accessed value is changing every year.