Rental Income For Attached In-Law Apartment

My father sold his personal home in 2020 and used his personal funds to build a small in-law apartment attached directly to my house. He lived in it until his death. Upon his passing in Feb 2023, my wife and I inherited the attached apartment, as it was attached to the house we live in and have always lived in. She began renting it out via AirBnB in Apr 2023. For the purpose of using Schedule E to account for the passive income, what value do I use for the space? The total value of the total house, including the apartment? Or, parse out the approximate value of the apartment itself? That would seem difficult as by our local town code, the utilities are consolidated into one bill (and always have been). Another dilemma: what timeline is appropriate to start accounting from? We bought the main house in 2017, the attached apartment was built at no cost to us in 2020, and we began obtaining rental income in 2023 (the same year we inherited the attached space).