It's not uncommon, actually, including in San Francisco, where there are many examples. As others elsethread have said, though, it's just not as easy or affordable as you'd expect. Here's a recent SF Chronicle article discussing it: "S.F. has nearly 16 million square feet of vacant office space. Why can't it become housing?", https://www.sfchronicle.com/business/article/S-F-has-nearly-...