Zoning reforms, building code changes, and other rules to create a finer mix of densities and uses in currently single-unit zoned neighborhoods
While your other ideas (antiparochi/"Seattle Swap" and straight-up zoning reform) are good and necessary, there are some issues with the idea that single stair apartment buildings are a required or even important ingredient in densifying our cities, at least here in the USA. (Canada is in a subtly different position, as their building codes are even more restrictive than the US IBC is on this front.)
First and foremost, we have the issue that single stair buildings of 5+ stories require aerial fire apparatus as an integral part of their egress operations concept. This is something that's handled through per-building site plan analysis in Europe (Germany in particular), but with the less maneuverable stock of aerial apparatus we have right now in the US (adopting European bucket-style aerial units would indeed help somewhat, but that's a whole another kettle of fish), this requirement imposes rather undesirable spatial constraints on the site planning of neighborhoods. It also requires undergrounding of power lines, which while desirable, is a cost adder to infill construction, and interacts badly with overhead-powered transit lines. (I wonder if the Europeans use insulated booms/buckets on their aerial fire apparatus to work around this, similar to how power company bucket trucks are built?) Aerial apparatus may also not be widely available enough to be part of a "first due" rescue company response in suburban jurisdictions that only have a limited number of extant buildings that require its use.
With that piece of site planning out of the way, we get to the issues with the building itself. First off, while the Seattle single-stair amendment to the IBC in particular represents a strong effort to provide compensating controls for the lack of a second stair, it is unclear how they'd perform in a fire situation since I know of neither tests nor detailed modeling that substantiate them, just the relatively "back of the napkin" justifications provided so far.
This is something a local permitting authority (AHJ) could resolve fairly simply, though, as NFPA 101 (the Life Safety Code) provides a *performance-based option* the AHJ can use as a basis for plan review, where the architects are required to submit a set of fire scenarios and corresponding model results to the AHJ along with the building plans. In particular, NFPA 101 22.214.171.124 requires an analysis to be performed for a scenario that involves an extremely fast-growing fire that by its nature, blocks an egress means from the building (such as a car crashing into the main entrance to the building at a high rate of speed then immediately erupting into a fireball, or a bucket of solvent getting accidentally kicked off the roof and down the stairs, then igniting) while assuming that all interior doors are open at the start of the scenario. If a maximum-permitted-height single-stair building built to (say) the Seattle amendment package was able to provide a "no casualties from smoke or flames not intimate to ignition" performance in a well-validated whole-building smoke/fire model exposed to this scenario, that would be a major step forward in establishing that pushing this needle is indeed reasonable to do, at least in places where the problem I'm about to mention doesn't rear its head. This methodology also provides an evaluation path for the legacy NYC single stair amendments (which, while requiring the building structure to not be combustible, don't provide any compensating life safety controls for the occupants).
That other issue is the problem of safeguarding that single stair from smoke, which is a key compensating control in the Seattle single stair concept. To that end, the amendments provide two options: either an outdoor stair, or pressurization of the stair similar to how it's done in high-rises. However, neither of these options are all that practical outside of climates that are mild to warm year-round (namely the Southern US, the West Coast, and our various tropical islands). Outdoor stairs represent a significant snow and ice clearance difficulty in cold, snowy climates atop the hazard of abruptly having to egress into subfreezing weather (ask anyone who's lived in a Montreal plex or maintained a building with exterior corridors in a cold place before), while pressurization's devil is in the (mechanical) details.
In particular, to avoid building damage (frozen sprinkler lines and risers or even frozen plumbing pipes) or stairwell pressurization system malfunction (due to unwanted air flows) in cold climates, you need to *temper* the incoming pressurization air, something that imposes an exorbitant heat load on the building HVAC system. Even reaching a 50F air temperature in 0F outdoor conditions (roughly 99% winter design conditions where I live) requires massive BTU capacity, to the point where the heater for stairwell pressurization tempering is over twice as large as the heating plant for *all common areas of the building* under normal circumstances, based on my own "back of napkin" work. A heat exchanger could be employed to reduce this load, but could also pose controllability issues in situations where the system is actively exhausting elevated temperature air from the stairwell, and I haven't been able to find any use of such in my admittedly limited research.
This massive HVAC load translates directly into a massive *standby power* load on the building, which since we want to avoid a fossil fuel generator set (what good is it to build a Passivhaus and then stick a diesel engine on it?), means we need massive batteries. Massive enough, even, to trigger the IFC requirements for energy storage system fire control and suppression, which include sensitive (hinged panel -- rupture panels aren't sensitive enough to prevent building damage here) deflagration venting for the battery room, as well as a sprinkler system built to the equivalent of NFPA 13 Extra Hazard water delivery requirements (or in other words, six times the GPM per square foot that residential fire sprinkler systems put out). And we have to have enough water supply (flow and pressure) coming from the utility to feed that sprinkler system, wherever it is in the building, _without_ recourse to a fire pump, as that fire pump would mandate we need a generator _anyway_, defeating the point of the exercise.
Of course, all of this presumes that the stair pressurization system is correctly designed and the building as a whole is correctly maintained so that the stair pressurization system performs precisely as designed. Stair pressurization systems in their natural habitat (the high-rise building) are notoriously sensitive to building and environmental conditions, as well as to alterations to the building that increase air leakage rates into the stairwell or change the functionality of other air handling components.
However, even though single stair reform isn't a panacea, there are ways to arrange our buildings to provide many of its benefits while still meeting North American egress requirements. In particular, Brynn Davidson @Lanefab has done good work in this space, designing apartment buildings that fit on to 50' wide lots (or even narrower) while providing two egress paths from and two visible aspects to all units, using an approach that has units opening to two stairwells: one centered at the front, the other in the middle of the building's rear. It's also possible to build the equivalent of single-loaded buildings using a multi-stair strategy that feeds adjacent stairs directly from the units instead of relying on corridors to connect many units to a small number of stairs.
P.S. I'm posting this here since I haven't been able to find a way to reach folks involved in the single stair discourse without tangling with Twitter, which I have no desire to do.
The NIMBY problem is inevitable when a freehold of a title id made available for a parcel of land. This negates the idea that the land, as a resource, like the minerals beneath it, are a 'common', that should be enjoyed by the current generation. Inheritance laws allow the inequity to be carried forward from one generation to the next. Secondly, it arises as soon as planners insist on consulting neighbours before planning approval is provided, institutionalizing dog in the manger privilege. Thirdly, once one has this sort of segmentation it extremely difficult to change it. Your draw the map, you are stuck with it. Given this structure, there is always going to be an underclass, no matter how hard they work or how meritorious they might be. They can aspire to enjoy what they see others have, but they will never get it. All of the reforms recommended here need to be vigorously implemented plus some. Sad to say though, it amounts to tinkering at the edges. Its like pushing the brown stuff up the hill with a pointy stick.
Fortunately, those who are powerless and often victimized, both intentionally and unintentionally, by members of dominant groups have ‘‘adopted the dominant] group’s ideology and accept their subordinate status as deserved, natural, and inevitable, at least on the surface and until they decide to riot.
Its interesting to reflect on the fact that 90% of urban Chinese own their residence and that subsidizing rent for those occupying smaller residences is seen as a viable way of moving the economy towards a situation where local consumption drives production.
Its going to take a lot of Yimbyism to turn this around.
The answer to any problem similar to the housing crisis in California is nearly always "all of the above", but I do think the emphasis on trying to shoehorn more people into existing single-family neighborhoods may be wasted effort. At most, you'll get a few thousand additional units per year built across the state, but that's against an unmet need that goes into the millions. If all of the same political, economic and social energy currently being applied to fighting with NIMBY's were instead directed towards constructing new high-rise neighborhoods around hgh-capacity transit nodes in formerly industrial, waterfront areas of the major cities, could the problem not be more quickly solved?
There are 100 square miles on either side of the LA River, probably close to that along the SF Bayshore even subtracting conservation areas off limits to development, still more in San Diego and Sacramento, a big part of these areas being composed of one-story, "industrial" buildings way past their most productive days. A high-rise residential neighborhood contains about 50,000 people per square mile, so if there are 200 square miles around the state where you could do this, that's enough housing for 10 million residents, taking California from 40 to 50 million people, with very little impact on any single-family neighborhood, anywhere. And using waterfronts, you can build parks and trails on one side, and heavy rail transit towards the other, and you have a great place to live as well as quality open space and transit amenities for nearly communities.
Would each square mile of that be an extremely heavy lift? Yes, of course. But is it a heavier lift than trying to shoehorn duplexes into single-family neighborhoods full of wealthy homeowners in Palo Alto or Westwood? I dunno . . . but success in the former means homes for 50,000 people, success in the later means homes for a few hundred. But all of the conversation is still about the latter.