One can try to approximate the actual value.
1. What do similar objects cost within radius x?
2. Standard land value to determine the pure land value (agricultural land is usually not worth much)
3. Cost value method for approximate determination of the value of the building. Here, it must be checked whether the parts of the house to be renovated have exceeded their calculative lifespan or not...
In nature conservation, it must of course be considered that if a building is renovated, certain requirements must be met depending on which species live there, such as installing nesting places for birds or bats. Certain work on the house may only take place during specific periods of the year... And yes, the building authority does come by and looks for bat droppings... That was the case with us.