I’m extremely pro-WFH for professions that can. I’ve been doing it for 10 years and it has only gotten better since others started to experience it and have empathy for what it means to be a remote worker. Just getting that out of the way before chatting more about hidden difficulties of converting buildings to residential use…
I can’t speak for European office buildings (your use of “flats” has me assuming you’re on the other side of the pond from me), but a large number of US buildings would either have to be 100% gutted back to the main supporting beams OR pulled down and rebuilt. Issue here is a combo of proper placement of utility lines (mostly plumbing) within the building and the added weight residential use brings rather than business use.
Large office leases here have a lot of control over how their floors are laid out, but floor planning normally takes electrical runs into consideration and will leave spaces like kitchens and bathrooms unmoved. Executive offices and other private interior spaces can be created/adjusted by making interior walls and tying into electrical connections already in a floor or drop ceiling.
Plumbing is a whole other monster and takes a lot more work. Not an insurmountable consideration, just harder.
The weight of residential living is one I hadn’t considered until someone pointed it out to me. In addition to all the additional plumbing needed (whose pipes add tonnage by the time you’ve converted a building), you also have to consider water within those pipes, and if a lot of people run their kid’s evening bath around 7 PM, that’s even more tonnage, normally all in a similar vertical line because of repeated floor plans. A lot of corporate buildings here, esp older ones, just weren’t engineered for that and a lot would need significant remediation to support it.
I have way less to say about the super cancers… We did use a LOT of asbestos as we built up urban areas, though.
My answer won’t be as popular, but it’ll definitely get the attention of the right people on the customer’s side: charge HANDSOMELY for issues caused due to customers using the systems outside of agreemed to or published best practices.