Looking beyond the regulatory hurdles (others have covered that, plus it's being worked on), there are some real practicality problems: 1 - Windowless rooms ar

Looking beyond the regulatory hurdles (others have covered that, plus it's being worked on), there are some real practicality problems: 1 - Windowless rooms are heavily undesirable in residences but pretty normal in office settings. In many areas they are also illegal (all bedrooms must have natural light) - but regardless of legality they are shunned by most buyers/renters. Offices however have lots of windowless rooms in the form of supply closets, utility rooms, freight elevators, conference rooms, storage rooms, etc. This combined with the popularity of open floor plan offices means that office buildings are really deep dimensionally. You can easily find modern office buildings where a desk is 50+ feet from the nearest window. This makes them nearly impossible to divide into residences, because the resulting apartments are dungeon-like, with only the very outer reaches getting any significant natural light. 2 - Cost of conversion is high but the resulting product is worse than purpose-built. Keeping in mind the natural light factor above, a lot of office-converted apartments are less pleasant to live in than purpose-built residential apartments, but the conversion costs are very high, and the two opposing dynamics cannot be reconciled. The cost of converting an office demands a level of rent that it cannot achieve in competition with actual residential buildings. 3 - Utility usage increases in residential buildings. A lot. Electrical, water, and sewer needs in residential buildings are much more intense than in commercial buildings. This is non-trivial to retrofit - since additional conduit and piping has nowhere natural to go. Conversion projects often have to punch through the concrete floor plates in order to create new top-to-bottom shafts to run all of the additional utilities. This is intensely expensive. 4 - Commercial buildings have over-built elevators relative to residential buildings. Office workers move from floor-to-floor a lot more than residents, and so for the same floor area commercial buildings would have more elevators. They take up valuable floor space that can otherwise be rented, and removing them to recover the floor area for residential use is incredibly costly. Many modern commercial buildings are in fact built around their elevator cores, so removal may not even be feasible. There are some successful conversions - but as others have brought up, they tilt towards conversion of pre-war office buildings where, because they didn't have powered HVAC, there are more windows and no office is too far from a window, creating good opportunities to convert into actually pleasant spaces. Modern offices with their gigantic floor plates are a whole different story.