[walrus01]

It's been done but very often "class A" office towers lack things which are needed for residential use, like windows that open, balconies, locations in the concrete walls and floor slabs for water and sewage pipes. Even basic things like if you take a floor of an office tower and turn it into 700 sq ft condos, how are you going to ventilate the bathrooms? Retrofit can be very costly.

https://www.google.com/search?channel=fs&client=ubuntu&q=qub...

I have seen some instances of older 1910s to mid 1930s era office towers in NYC retrofitted to (very high end, very expensive) condos.

[failwhaleshark]

Yep. It's not practical. It's often better to bulldoze, rezone, start over, and build high-density vertical housing. It's totally SimCity. :)