@Anecdoche I think 'expropriate' is the correct term if the authority comes from the state I.e. a judge in a civil court ruling. But it could be that expropriation was the correct term as my situation was different to OP as it the local council who claimed the land on my behalf.
It was a top floor flat in a 3 flat Georgian semi detached conversion (weirdly not listed but the whole street was a bit of a mishmash). I had a leasehold with exclusive right to my flat and enjoyment of the communal parts of the property as long as they were 'accessible'. Basically as long as I don't need to trespass on someone's land to get to the communal part than it's shared access. The communal parts were front garden, rear garden and loft/roof. I bought it off a lady who was essentially bullied out of her home by the two neighbours below her.
The land in dispute was the rear garden which should have been accessible for my top floor flat by the side access gate. But the two flats below boarded it up 10 years or so before I bought it. The freeholder was the council and the middle flat was a council tenant. The couple in the basement were even bigger arseholes than me and boarded up the side access gate and put up a fence 'giving' the middle flat a quarter of the garden land... somehow they convinced the lady in the middle they owned the garden when they bought the basement flat.
I ripped down the boarding and fitted a gate to the side access and dismantled the wooden fences partitioning the rear garden and stuck a shed slap bang in the middle of the garden with a camera, an alarm and a sign warning against criminal damage.
Originally the council took my neighbours side but the court took my side and made the council 'expropriate' their own communal garden land back from the tenant in the middle flat and the leaseholder in the basement so that I could continue my very legal enjoyment of that space.