Because the improvements were personal (made during your period of personal use, and before you held the property out for rental) you don't include them as rental expenses.
Instead, they are part of the adjusted cost basis of your home. When you place the property in service as a rental, your base for depreciation is the adjusted cost basis of the property, or the current fair market value, whichever is lower at the time. The adjusted cost basis is the price you paid plus the cost of improvements.
So, let's assume you purchased in 1999 for $100,000 and paid $30,000 for improvements in 2017. The fair market value in 2018 was $195,000, including $25,000 for land. Your basis for depreciation is $105,000 ($130,000 adjusted cost basis minus the value of the land).
By the way, you use fair market value, which is not always the assessed value. FMV is what it would be worth to an unrelated buyer if you listed it for sale.