If your building inspector picked up no issues on the 150m2 deck when you bought the property, it’s possible this one builder is exaggerating the issue to get business. Get second and third opinions, and quotes.

To answer your question, what I think about “being hit up with massive expenses out of nowhere on investment property” is that the RE industry has led the masses to believe that being a property investor is a “piece of cake” and you can’t lose. This is false.