dg wrote:

"...Why can't such an example be used as proof that a particular
construction method will be adequate and safe for the next 100 years
and thus comply with the Building Act?"

I'm no expert on the topic, but isn't it all to do with changing
standards. You could try to argue that the standards to which a 100
year-old building was originally built are 'adequate', but they just
won't meet the current requirements. Take building insulation, for
example and think about what you got with a 100 year-old building
compared it with today's requirements.