People better be on guard, how long will it be before they start taxing the spare seats in your cars?!
I see most cars have four or so spare seats and only one occupant, It will benefit the environment thought, so not necessarily a bad thing.
Unused clothes and shoes in the wardrobe, uneaten food in the cupboard, or a tax on the fill level of your food waste bins... unused parking space next to your homes...
Why do you think they relaxed the building regulations on extensions and conservatories, they intend to tax them later.