Can I add improvements to rental property that were done in prio

I sold a rental house in 2018 and I have been depreciating the value of the house since I started renting it but never included the value of the improvements (around 30K, including hardwood floor and kitchen remodeling).  How can I add the value of the improvements to my total cost in order to increase the basis? Is this allowed? Do I need to go back amend prior tax returns? 

 

Thank you so much for your help!