The site has one value. And a mom & pop operation is just as likely as an investor to own or buy an on going parking business. There are are lot of syndicated parking lots in towns across America and sprinkled among them similar one owner parking lots. Often they are developed later after the site has ripened to a higher and better use. Say a parking lot may be bought to build a downtown convention center and hotel with parking. They may build a level or two below ground as parking and the first floor is the lobby. Perhaps some shops with all the rooms above. But that is a future unknowable event.
Don't let HBU get off into the weeds speculating upon some future use. HBU is here, now, today. And it is either "as if vacant" or "as (is) improved". Every house or improvement has an ideal improvement, but every ideal improvement is almost certainly a brand new building - which renders HBU rather meaningless if trying to extrapolate to today. HBU is simply asking "Are the property improvements contributing value to the land as if vacant." No matter how much functional or external obsolescence exists so long as it contributes value to the land, it is the HBU today.