As far as I can tell, the actual house is missing. This is the outbuildings and tunnels and associated non-dwelling rooms only, and planning permission exists to build a house on this land. I suspect you won't get planning permission to use what is there as a house, but instead are required to replace this with a house according to the approved plans.
Perhaps they split the previous house out into its own plot and sold it separately.
Perhaps they split the previous house out into its own plot and sold it separately.