Couldn't we follow the model Spurs did with WHL - Build 3 sides of a new stadium on the adjacent land / car parks behind North, leaving the North Stand insitu and therefore the existing stadium intact during 75% of construction works, then demolish the existing stadium, building the North Stand back facing the other way and completing the 'ring' of the new stadium. It would mean us either playing elsewhere or in a 3 sided stadium for the duration of North Stand works but if we have a capacity increase then a newly configured 3 sided stadium wouldn't give us far off the existing capacity at the minute.
Sounds expensive but the costs of building new will probably compare over a 25-30 life cycle with trying to thoroughly overhaul the existing stadium and dick about trying to cantilever another tier over the South Stand plus routine maintenance on what in parts will be a 60 year old building.
Any new design should incorporate Safe Standing. The game and stadia design have improved massively since 1989.