The client wants me to value the improvement only; the house will be sold and moved to another location.
If sold off you are dealing with personal property. Check your yellow pages, there has to be a house mover somewhere within 500 miles of your location. Contact them and get the low down on their experiences.
In 1997 my area had a monstrous flood, the aftermath included the relocation of a lot of homes. Some were sold and moved off, some were moved by the owners. Values ranged from a few thousand dollars to a substantial sum. Two houses in particular show the variety in value.
#1. A large very nice, newer ranch style, appraised on site for over $1 mil was sold for just over $100,000 and moved to a new location. There the new owner got quite a house for the cost of the move.
#2. A house mover was contacted by an individual to have a house moved off a large residential parcel. The mover quoted a fee to move the house to the outskirts of the city, thinking the owner had a new site picked out. The home owner accepted the bid, the house was moved. When they had it at the city limits, the mover called and asked where the owner wanted his house moved to? The owner said, what house, I asked you to move it off my site, it's off, the house is yours....
So, you can't value the site and offer contributory value for the personal property. You have to go to the market, wherever that may be.
It's going to be more difficult but it will be an educational effort.