In 2017 we rented out our principal residence (change in use). We used FMV as the cost of the building (used appraisal). How about FMV for land of the rental? estimate?

I cannot find any information regarding the split between building and land. The sale agreement and tax assessment doesn't provide this information. The townhouse is a condominium unit. It  has a small backyard. Should I estimate the value of the land as a 20% of the overall FMV of the appraised building?