Think carefully about what exactly it is that you would like to commemorate. Is it the house and the history of its occupants or is it the architect of the building and its architectural design? Is it the entire cemetery, a community of people buried in the cemetery or the life of one individual buried there? Or is the event related to a specific disease that took the lives of many in that community?
Putting up a plaque or marker to honour the recently deceased is a memorial. Generally, heritage plaques and makers commemorate the life and work of an individual who has been deceased at least 15 years.