An extract from:
https://www.joe.co.uk/sport/safe-standing-qa-91188
How many people can you fit into a rail seating section in comparison to a conventional stand?
“There is no ‘one size fits all’ answer to that question. And there are many factors that can determine what the answer would be in each individual case. These include national regulations, the amount of space available between the rows of rail seats and the size of the stadium exits and concourses.
“In a country where the national regulations permit it and a stadium’s exits and concourses are large enough to handle the number of people concerned, you could be looking at as many as two people per seat space (in Hannover, for instance, 5,700 fans stand in an area of 3,000 rail seats).
“In the UK, however, seating rows tend not to be as deep as they are in, for example, Germany and in existing UK stadia the exits and concourses will have been designed to cope with no more people than the number of seats in the stand. In retrofit situations, therefore, achieving more than one person per seat may present a challenge – albeit not insurmountable.
“For new builds, however, the situation is different. Whether architects are designing a new stand or a whole stadium, it is clearly possible for them to design exits and concourses large enough to cope with more people than there are seats. This is precisely what Tottenham have done for the new White Hart Lane “with sections of the seating bowl designed to accommodate areas of safe standing if the current licensing regulations change” and potential built into the design for the 61,000 capacity to “increase by c 1800 were legislation to change and safe standing areas be approved.”
It does sound like it is possible that safe standing could lead to an increase in capacity but there are a number of steps required first:
1) Government legislation
2) Stadium design has to be configured in advance in such a way to make 2:1 possible (otherwise in my opinion no advantage - although others probably disagree)
I am sure any designer will be aware of this even if we are only vaguely familiar with issues.