Filtering: The Opposite of Gentrification

While filtering is not some panacea that cures all housing ills, it is an important part of how housing markets work. Allowed to take place, it contributes to providing housing at all levels of income. The more a city impedes the process by restricting growth, the more its poorest areas will gentrify without the offsetting creation of less expensive housing over time. 

Buildings, like anything else, are expensive when they’re new but depreciate over time. Architectural styles change, wear and tear accumulate, and buildings become harder to update with the newest amenities. Here is where we find filtering: aging units, originally built for the wealthy, become more affordable over time. What’s difficult to see is that filtering occurs simultaneously with gentrification. Every “gentrifier” frees up their former unit for someone slightly less well-off. That person, in turn, also frees up a unit and so on down the line. This process is akin to a game of musical chairs.

But for the game to work for everyone, chairs must be added, not taken away. Cities must allow additional units to be built so that the housing stock expands over time. Research suggests this is a necessary condition for filtering to take place.