I haven't looked at the stand cross sections for a while, but from memory I didn't think there was much space between cantilever upright and back row for any additional rows on the sides.... unless that has changed since.
However, both end stand roofs are supported by full width trusses and therefore additional rows could be added (sightline permitting). I think Binman did a mock-up adding a few rows to the south stand. If we ever acquired the (air)space at the North Stand, that would be a relatively straightforward addition too.