stop it from deteriorating

It seems that constant maintenance and repair is the effort, here.

But while we all can agree plumbing in some modern power and modern plumbing and modern non-lead water pipes is probably verboten, where's the line between repair and addition? Are residents allowed to rebuild damaged portions as per the old appearance using new stone and the old methods? Or must they maintain "the South wall, near collapse from damage with those other people, kept in its teetering state for the sake of preservation as it was on this arbitrary date" or something equally bonkers? What's the arbitrary target they are to repair and maintain towards?