I wonder how many of those old 'taxpayer' one story buildings were actually built, from the get-go, with the structural provisions to go vertical eventually? If I understand the history correctly, they were made to be one story to keep their property valuations down during the depression...some owners back then, though, must have been thinking about the bigger-picture, right?
(EDIT: I think some of them even were multi-story originally and then had their upper stories pulled off. Those should, in theory, have the columns/foundation to receive that load again)