For a lot of the office buildings I've been in, there aren't that many toilets per floor. Its also different when you've got some toilets that are often unused compared to people running laundry, cooking, bathing, etc. Very different demands on the plumbing system.
You also then had everything pretty much isolated to two rooms for an entire floor meanwhile now every unit is going to have a separate kitchen, a bathroom (or two, or three), a laundry room, etc.
And you're going to need a good bit of engineering studies done before you start cutting that many holes in the floor.
But do you really have to cram in as many residents as you could with a purpose-built tenement? There must be ways to keep headcount in the range the infrastructure can support and still provide a lot more housing than just leaving them as empty decaying offices owned by the last one holding the bag. Intersperse flats with windowless storage units (you have a depth problem anyways), low density commercial use like workshops with live-by flats and so on. Large units designed to attract high squarefeet/low headcount tenants, not bunk bed families. Add regulation only as a fallback limiter. Don't let perfect be the enemy of good.
> And you're going to need a good bit of engineering studies done before you start cutting that many holes in the floor.
You can Swiss-cheese a pan and deck concrete floor with core-drilled holes, the important thing is GPDR scanning before coring to avoid the pre- or post-tension cables embedded in the concrete.
Ok, but some extra plumbing (and whatever sorts of engineering studies referred to) and electrical work surely can't as expensive as demolishing and rebuilding a whole building.
These seem like extremely solve-able problems.