Single stair is one of the reforms I'd most like to see.
At the time 2 stair requirements were adopted it was vital, with devastating urban fires a common occurrence. We have so many new options for both preventing fire and keeping evacuation routes accessible for hours that it's no longer required.
The regulation has a huge impact on the layout and form it's possible to build, and I think it's a huge driver of the visceral reaction against apartment living in the US and Canada.
Being able to build 4-8 storey apartments on a single lot with a central stair where every unit has windows on at least 2 walls would be a game-changer for north american urban spaces and a pathway out of the housing crisis.