> This is a general argument assuming units being arbitrary.
Well, no, it doesn't assume units are arbitrary, it assumes units are fixed square footage, which they are not. Under most regulatory schemes, there is a practical minimum size or a habitable unit, but a pre-existing area zoned for detached single-family units exclusively is unlikely to be comprised of single-family units that happen to also be the minimum square footage for a habitable unit.
The same concept, at a minimum, would need to be extended to units. This is what I mean by assuming it’s arbitrary. It’s just redundant to say that, yes, obviously you need an incremental increase in units, sqft’s, footprint, vertical footprint, etc.