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.
It looks quite clear that they have split/subdivided their plot. In Australia this would be called a battle-axe property. The listing talks about "land associated with 123 Street Address".
They are basically selling off the far back garden of the main house. A long thin access road, the pool, the glasshouse/conservatory, the workshop, the tunnels, and the planning permission.
Perhaps they split the previous house out into its own plot and sold it separately.