[imnotreallynew]

I’ve read opinions, perhaps presented as facts, that transforming commercial buildings into residential units was often more expensive than tearing down the whole building and starting from scratch.

If that’s true, there seems to be a large opportunity available to those who can figure out how to transform those buildings in a way cheaper than what’s currently possible.

[shiroiuma]

I seriously doubt that it's possible, unless you can get the city/state to change the building codes dramatically for the worse. Residential units need various things that commercial buildings don't, like windows (for each unit, not just at the periphery of the huge building), lots of plumbing so everyone can have a kitchen and bathroom, etc. Retrofitting buildings (to be very different than before, not just changing some fixtures and keeping the overall design intact) is generally more expensive than just demolishing and building new ones that are properly designed for that purpose. Just look at cars: even restoring an old car costs a fortune in labor costs, because it's far less efficient than building a new one on an assembly line, and that isn't changing the car design substantially. Imagine what it would cost to transform a 1950s car into one that meets all 2023 safety and emissions standards; even if you can reuse an existing engine/transmission, there's a ton of custom work needed, both mechanical and electrical.

[h0l0cube]

Or just tear them down and build something new.

Main issue with office space to apartment conversions is the deep floor plan that makes access to sunlight difficult. Retofitting plumbing and isolated fireproofing probably adds more complexity. In terms of malls though, it's perhaps less of a problem to turn these into low-rise residential.

But at the end of the day, it's just land waiting for re-use. I'd be happy for these spaces to become parkland and forest. Or maybe there will be something in the future that needs a lot of land but benefits being located somewhat closer to populations?