It's difficult to say there's any particular one, because a building is made up of so many different parts. Each one is sort of a de minimis cost, which is how these costs increase. You say, “Well, this is just a little thing. It's just a little part of the building. What does it really matter?” An elevator, for example, is only 2% of the total cost of construction, but you find these costs in the entire building. It's hard for me to isolate just one or the other. There are some things that I know about, such as ventilation and plumbing, that go a bit overkill. I know a lot about elevators and a lot about stairways. You have separate standards for windows. It's every part of the building. It's hard to isolate just one.
I guess my general recommendation would be to go through the building codes systematically, look for differences in the global standards, which tend to derive from the European ones, and try to fix them all, ideally. That's really the task of a lifetime, but I think it needs to be done. It's hard to pick just one. I know the ones I've studied, but that's not to say these are necessarily even the most important ones. They're just the ones that, with limited resources, I've looked into.