See IRS Publication 946 at https://www.irs.gov/pub/irs-pdf/p946.pdf On page 33 of that document at the top of the 2nd column it lists Residential Rental Property at 30 years. You'll note there's a subscript "1" for that line. Reading note 1 states it's 40 years for property placed in service before Jan 1, 2018. So your property improvements that are classified as residential rental real estate that were placed in service on or after Jan 1 2018 are depreciated over 30 years.
