Legislation on the option to tax underwent some changes in 2008 and since then it has no longer been possible to opt land and buildings separately.
The option to tax in 2011 covers the whole of the building, and the land under the building and within its curtilage (the land immediately around the building including, for example, forecourts and yards). If your interest in the building is restricted to one floor, your option to tax will still cover the remaining floors of the building.
I’m afraid if the building is demolished or destroyed, the option to tax will still apply to the land on which the building stood and to any future buildings that are constructed on the land.