I would say that you pick up where you left off on the original depreciation calculation started in 2001, except in the rare case where the FMV in 2023 is less than what it was in 2001.  (That just didn't happen in Las Vegas real estate market.)  That calculation runs only another 17.5 years and should be kept separate from the improvements from 2012 through 2022, which can run 27.5 years.  In other words, you'll be tracking two assets, not just one with combined amounts.  But maybe there is a special rule for property that goes from business to personal and back to business.  If so, where did you find it?