When you buy a family plot you don't 'own' the grave you have the use of it. If anyone is stuck for a last resting place having lived the athiestic high-life the newest (non-denominational) solution is a green burial with a 'wrapper' of hessian or a simple recycled compressed paper coffin, all in the field or wood of your choice with a tree planted on top if you wish. We discussed this on AAM some years ago. I put some contact details for Irish green burials.
As long as you don't bury a corpse adjacent to the human water supply system you can legally dispose of remains anywhere, including your own back garden.