I placed a former primary residence into a rental in 2016 and started to depreciate the property that year. I sold the property in 2018. However, I forgot to add the cost of the swimming pool I installed in 2015 as part of my cost basis on my 2016 tax return. Can I add the pool on my 2018 tax return, with an install date prior to 2018, that will increase my cost basis or do I need to amend my 2016 and 2017 tax returns first, showing the increased cost basis which includes the pool?