In 2019 I purchased a one-acre property that has two homes - a 2 bed/1 bath, 1176sq.ft. home that has been (and remains) a rental, and a 450sq.ft. studio home (like a guest cottage), where I live. The property cost $335,000. The renter has primary use of about 1/3 of the property. How do I determine the value of the rental, given that it is actually the main residence, whereas I am living in the smaller home but have access to more of the land?
In my opinion, first, I would look at real estate tax statements to see if any breakdown there that can be used.
If not, you might want to consult a real estate person. Most accurate would be to get an appraisal of the property with breakdown between the houses and use of land.
Absent that, I'm not sure what else you would do.
Regarding the land value, this does not factor into the allocating the valuation between your residence and the other residence on the property. TurboTax is asking you the basis/cost of the property because it will guide you through depreciating the main structure. Land is not depreciable, so its value is not considered at all when depreciation is calculated. Echoing LudwigVan_fan, the safest and most accurate way to determine the value of just one structure on the property is via a qualified appraisal.