I was told from someone who knows someone at the club (take it as a pinch of salt) that it will add 10k seats, will take around 9 months and will be done similar to Liverpool’s so the stadium isn’t affected during the works.
The same source told me that they will move the tunnel to the East Stand and that where the tunnel/changing rooms are currently, it will become a museum.
As I say, thought I’d pass on the rumour, even if not credible.