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Converting older office buildings - say, 1950's and earlier - is often feasible, it's the newer ones that can be problematic. Most people don't want an apartment without a window (and often fire or occupancy codes require a window). This isn't really an issue for older building stock, as they were constructed when air conditioning wasn't as prevalent, windows provided ventilation, and window light was used to supplement office lights.
Modern office buildings don't worry about windows for either ventilation or light, so each floor can take up a massive amount of space. If this happens to be a long, thin space, you could put in some apartments - but a lot of the buildings are more square.
How do you handle that? Do you make each apartment really long and thin? If so, do you put in a hallway on one side that eats up precious space and does nothing other than keeping you from going through each room in turn? Or do you make it so you have to pass through each room to get to the end?
If you have the pass-through-each-room style, then which room should be the end room? Traditionally the living room gets the big windows, so you can entertain guests, but that leaves you passing through bedrooms to get there. If you put a bedroom at the end, then only one person/couple gets the light, and you're still potentially walking through the second or third bedroom.
You could make the apartments more square - but these are massive floors, sometimes taking up entire city blocks. And as I mentioned, often code requires windows, so what do you do with the massive space in the center? Do you make each apartment wide and long - those will be expensive and won't help the affordability crisis. Do you build in common areas: say, put in resident storage units every 3 floors and a gym every 5 floors and toss in some community spaces? That's great, but those common spaces will need housekeeping and maintenance, which raises ongoing costs. You can put in office space, but most people don't feel comfortable having those on the same floor, and it raises security concerns for the residents. There are a couple places that have put in a giant light well in the center, but that's expensive and makes the resulting apartments expensive too.
Conversion tends to work better with older building stock and while that works fine in some places, what do you do in cities that don't really have a good supply of older buildings? The supply of 1950's era office buildings is certainly limited in places like Los Angeles or Phoenix.
