"Are you familiar with the Texas doughnut?" Chris Gannon, an Austin architect, asked me recently. The question wasn't about confections. He was riffing on apartment construction, explaining why America can't seem to get behind the kinds of buildings that he and other pro-housing advocates want to see.

The "Texas doughnut" is shorthand for a large apartment building, often spanning an entire city block, where a ring of street-facing units conceals a parking garage in the middle. In growing cities across the US, doughnuts are everywhere. In Gannon's eyes, they're clumsy solutions to our housing woes: The imposing structures offer little outdoor space and require a lot of precious land.

It's much harder to find smaller apartment or condo buildings, the kind that might fit snugly between a single-family home and a local restaurant. Cities are full of these little parcels, but "missing middle" housing — bigger than a single-family home but much smaller than the popular doughnut — just isn't getting built. This reality is especially frustrating for pro-development yes-in-my-backyard activists who have spent years rewriting laws in hopes of encouraging denser, more-diverse housing — some baby carrots to go with all those doughnuts.

In their quest to build more and bring down costs, YIMBYs have identified one overlooked hang-up: stairs. Yes, the humble stairwell is an unlikely but powerful foe. More specifically, they're taking aim at a widespread rule that requires almost ever.