They cost a lot of money to maintain and you cannot just let them decay because they come with an obligation that you maintain them. This means not only a vague general obligation but the monument authority will make specific specifications what has to be done and what can't be done. You'll have to get approval for every modification down to little details like the very specific type of window frame and such things. In some areas these obligations can also include that you open your premises for the general public once a year for the National Heritage Day.