How do appraisers account for a bowling alley in the basement?
Or some other unusual feature, like a helicopter landing pad, 10,000 gallon aquarium, basement basketball court, or dozens of other unusual — and therefore hard to estimate — home features?
If it’s possible, the preferred strategy in such situations is to find another home that otherwise qualifies as a Comp (“Comparable Sold Property”), and also has the same unusual feature/amenity.
Once you have two homes that each have basement bowling alleys (or some such), the feature gets cancelled out.
Then, the remaining adjustments focus solely on where the two homes — the Comp and the subject property — differ.
Of course, the more unusual the feature really is, the less likely you are to find another property that has it.
P.S.: I personally know of exactly one home that has a basement bowling alley (or did): the White House.