I was flicking through 'Liverpool Heritage Walk' before, and I read about Roscoe memorial gardens. Saying that William Roscoe was buried there, but I assume the grave (as a headstone above ground) no longer exists....?

It got me thinking about other burial grounds in the city that have been built upon. e.g Necropolis, and Church Street cemetery, also the one that was on the corner of Paradise Street (where Edge hill mole was buried).
How do people get the rights to build upon graves?