Hello Colleen
Regarding a Headstone, usually the owner of the Cemetery will be able to advise about their Communal Grave Rules and also provide you with the written Rules on the errection of a headstone or memorial, also if and where permissable and on what type of graves, along with their Admin Application Fee cost and also who they will permit to give you an estimate and do the work.
We used a reputable long standing Monumental Mason local to the cemetery and familiar with its Cemetery Rules to provide us with a price & costs breakdown.
Some Masons include the Admin Application Fee (ours was £140) on top of their costs, other Masons get you to pay it direct to the Cemetery Owner.
We had to have two specially machined stainless steel bars (that don't corrode like steel) concreted in, down about two feet in depth and that go up inside the headstone. The Plinth, Base and Headstone are specially machined to fit over the bars.
Ask to see the Cemetery Authority's current Rules and Charges too.
If you are permitted, but don't follow the Cemetery Rules as to how, where and what you can errect, some cemetery owners have certain rights regarding Headstones that don't conform.
If going on an actual grave, many insist on the grave reference number engraved at the base of the headstone, a very good idea when visiting any cemetery to see this was done.
One occasionally sees on a Headstone 'buried near this spot', or 'buried in this cemetery', suggesting that in the past, some cemeteries allowed headstones to be added later, or the exact grave location is not known.
Mark