Those BuildZoom maps of "no build zones" across the country are, to a large extent, maps of areas zoned for exclusively single-family homes. Once they fill up with single-family homes, that's all she wrote. None of them can ever be replaced with anything else, except maybe a larger single-family home. No duplexes, accessory dwelling units, small apartment buildings, or corner stores.

A neighborhood built to a finished state is wired for decline, because it will never be as nice as it was the day the buildings were brand new. The neighborhood in the top photo (median home value $86,400) is functionally the same as the neighborhood in the bottom (median home value $466,600). Both are suburban subdivisions full of exclusively single-family homes. Both were built to be middle-class communities, a little slice of the American dream. The only difference is one was built out in the 1960s, the other the 2000s. Shall we place bets on what demographic will live in that bottom neighborhood in the 2040s?

Decline is not natural. It's a function of the way we develop in America: all or nothing, and to a finished state. A monocultural neighborhood — all the same type of building, all built around the same time — is wired for decline because if market tastes and needs shift, it affects every property in the same way. It's like investing your whole net worth in bitcoin. A neighborhood with a diversity of buildings of different ages, sizes, and uses within a neighborhood has a diverse portfolio — it has hedged its bets. In such a place it's possible for renewal to be an ongoing process.

To the extent that renewal is possible at all in single-family-only neighborhoods, it occurs in a way that is destructive of socioeconomic diversity: demolition of modest homes, only to build huge homes on the same lots, with no increase in total housing units. Portland, Oregon, is growing and has a strong economy. But if your budget for a house is under $400,000, the city is shrinking before your eyes, as an eye-popping animation by the Sightline Institute reveals. And a major culprit is that neighborhoods that might incrementally densify and take some of the pressure off the housing stock can't do so because of single-family zoning. Instead, the city has had several record years of residential teardowns.

Hold on, you say. Many neighborhoods that have suffered blight and disinvestment actually do have a diverse mix of building types and uses. Why wasn't this enough to hedge against decline? There are historical answers that have to do with mortgage lending discrimination and racial redlining. There is also, I suspect, the fact that the prevailing development pattern has introduced enough distortion into regional real-estate markets that no place is fully immune from the waves.

Remember Rent Gap theory. As a developer with the finances to work at a huge scale, if I can exploit a massive rent gap — that is, dramatically increase the value of a piece of land and reap a windfall profit — why would I even pay attention to a neighborhood where the available opportunities are piecemeal, incremental investments? I likely wouldn't.

We have the fire hose or the vacuum. Nothing in between.

The political battle to re-legalize incremental change is uphill, maybe impossible in many neighborhoods, but those of us who want cities without deep disparities between booming and disinvested areas need to advocate for sweeping changes to how we regulate land use. The next increment of intensity should be allowed everywhere as of right — converting a house to a duplex, adding an ADU, putting in most types of Missing Middle housing.... A neighborhood should always be a work in progress.

4. Don't just calm the waters; nurture the reeds.

Even if we managed sweeping policy changes — removing regulatory barriers to small-scale development, a return to incremental patterns of public investment, and a zoning regime that allows the gradual, organic evolution of neighborhoods — it would be naive to think the magic of the market would lead to inclusive development in short order. A dam may destroy the ecosystem of a river, but blowing up the dam does not instantly restore it. Those who disproportionately corner the market on development capital, expertise, and political connections right now are likely to enjoy self-perpetuating advantages unless we actively nurture an economy based on small-scale, local wealth creation.