After reading your comment I'm wondering if we have read the same article. It's pretty self evident that adding an extra staircase takes up significant space as

After reading your comment I'm wondering if we have read the same article. It's pretty self evident that adding an extra staircase takes up significant space as everyone who has tried to design a floor plan will tell you. For larger buildings it's not just the area of the stairwell, but also the corridor that has to connect the 2 of them. I can also confirm that most apartment buildings in Germany, France, Scandinavia, Austria and Italy (that I'm familiar with) have only 1 stairwell (note that's different to exists, many have at least 2 exits). From what I have seen they apartments (like the article asserts) tend to have much nicer layouts with more light.