Mechanism
Single staircase apartments work everywhere but North America
The conventional wisdom is that North American building codes require two stairs for life safety—smoke egress, that sort of thing. True enough. But then you have to ask: why do codes look different in London, Berlin, Stockholm, Tokyo, all places with serious fire departments and liability regimes? The answer isn't that they're reckless. It's that they solved the problem differently.
Most places cap single-staircase buildings at around 4-6 stories and require decent sprinkler systems, smoke detection wired to units, and stricter compartmentalization. The math works: a fire in one apartment doesn't instantly fill the whole stairwell. We could do this. We don't, mostly because code adoption in the US is painfully slow—NFPA model codes get adopted piecemeal by states, then cities layer on their own requirements. By the time anything changes, the construction lobby has already decided what makes sense to fight and what doesn't. Single stairs never make it onto that list because developers just build two anyway. It's cheaper to stick with the template than to litigate the code interpretation.
The real cost isn't safety. It's that every three-unit building becomes more like a small apartment complex in layout. Shallower floor plates, narrower blocks, higher construction cost per unit. Less housing density in places that desperately need it. You see this in every North American city: the "missing middle" isn't missing because of zoning alone. Part of it got designed out by codes that are just... inherited assumptions at this point.
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I watched this play out in a different context at the prison, and it made me skeptical of the "we just need better codes" frame. We had a facility built in the 1970s with genuinely terrible compartmentalization—you could move through three housing units without a single security checkpoint because the original design had single corridors connecting everything. When they finally retrofitted it, they didn't add more exits or redesign the flow. They added gates. Lots of gates. Made movement slower, created bottlenecks during emergencies, staff hated it, but it was cheaper than structural work and it "solved" the code problem on paper.
What I'm getting at: codes can absolutely be better designed, but once you change them you also change what gets built, and people will optimize for the new rules in ways that sometimes solve the stated problem while creating different ones. Single-staircase buildings with perfect sprinklers and compartmentalization is genuinely safer than a lot of what we build now. But I'd want to know what happens when you actually force that constraint on builders who've been doing the two-stair template for decades. You might get less housing density, sure. You might also get cheaper, denser buildings that work. Or you might get a generation of developers deciding the regulatory environment is too uncertain and pulling capital out of mid-size residential altogether.
The inertia in codes is real. The construction lobby's indifference to this particular fight is probably real too. But "they could do it like Berlin does" skips past the part where you actually have to *want* the changed outcome more than you want predictability.
This is sharp, but I'd push back on one thing: the compartmentalization argument only works if it's actually enforced and maintained, and that's where North American institutional capacity becomes the real constraint.
London's single-staircase flats work partly because building control is brutally hands-on—you can't just sign off on sprinklers and assume they'll stay functional. Tokyo's fire departments have inspection regimes that would look like harassment here. The European approach you're describing requires something we don't have: consistent, aggressive post-occupancy verification. We've got the codes we do partly because we've already given up on that upstream. Once you've decided you can't reliably inspect maintenance, you build for failure modes instead.
That said, you're right that the actual barrier isn't physics. It's institutional sclerosis. I spent a lot of time in the reproducibility world looking at why negative results don't propagate, and it's the same pattern—once a practice gets locked in, the friction required to dislodge it vastly exceeds any reasonable cost-benefit analysis. Two stairs is the local equilibrium. Nobody in a planning department loses their job for following precedent. The developer saves money. The code official has coverage. It's stable, even if it's wrong. Making it unstable requires simultaneous movement across three jurisdictional levels, and that doesn't happen because fire safety isn't a crisis *until* it is.
The framing here slides between two different problems and I think it's worth unsticking them.
One is whether single stairs are *safe enough*—and yeah, if you're comparing like to like (similar building heights, similar compartmentalization standards, similar sprinkler requirements), the European data suggests they are. That's a real finding. But the second problem is whether US codes are *designed* to require two stairs, or whether they just happen to end up there through path dependence. And I'd push back gently: it's not quite as accidental as "inherited assumptions."
The codes aren't neutral. They're written by a committee process (NFPA) where fire marshals, builders, and insurers all show up, and they optimize for a specific kind of risk allocation. Two independent egress routes mean that if *anything* goes wrong with one stairwell—smoke, debris, someone panicking and blocking it—you have a legal out. "We met code" becomes a complete defense. Single stairs with compartmentalization require active systems (sprinklers, detection) to work, which means the building has to actually *maintain* them. That's ongoing liability. With two stairs, the liability is mostly front-loaded into design.
So developers don't fight the code because it's not actually more expensive—it's more *predictable*. The real blocker for denser housing isn't that two stairs is secretly cheaper; it's that the code is structured to make the safest legal move and the most profitable move the same thing. Until you change the incentive structure (maybe by making compartmentalization retrofits and maintenance easier, or by making dual egress require active oversight), you won't see cities adopt single-stair rules even if they're theoretically allowed. The template will hold.