If you go for a walk around a sought-after North American city, you’re likely to see all the usual things we associate with booming city growth: the towering crane, the beeping backhoe, the shell of that new apartment building looming behind the construction tape. But just because the footprint of your town is growing doesn’t mean your population is—even if every single one of those new luxury units is filling up fast.

And if you want to understand the reason behind that seeming paradox, we have one tip for you: start counting doorbells.

That’s because, according to a new article from Slate’s Henry Grabar, many of the cities that are adding new housing units are also among the fastest to subtract old ones from within the walls of their historic buildings, converting duplexes and triplexes to single-family homes and gradually draining the density from what used to be populous neighborhoods by design. But how should a Strong Towns advocate look at this phenomenon: as a disaster for communities that are short on affordable housing supply and too slow to build, or a natural and necessary part of the incremental development process?

Today on Upzoned, Strong Towns staffers Kea Wilson and Daniel Herriges talk it out. Daniel, a San Francisco veteran who’s spent a lot of his life in booming places, gives his take on the long-range consequences of strict zoning codes that make it near-impossible for buildings to evolve to the next level of intensity, but all too tempting to knock out a wall or two and remove that second kitchen. And Kea, a Rust Belt native whose own city has seen a wave of single-family conversions without the accompanying new housing boom, talks about why even cities with more vacancies than they know what to do with might not welcome the Disappearing Doorbell problem—at least, not until the desirable neighborhoods with the strongest development patterns can upzone themselves by right.