One of the common criticisms of new housing is that it's designed for rich people and that it does nothing to help the housing situation of average citizens. The YIMBY response to this is, "Well, yes, it does actually, because supply eases overall housing pressures and because of the filtering effect." This is the process whereby housing becomes gradually more affordable and available to people as new housing is built and vacancies are created. But most people don't like this explanation. It feels slow and indirect.

Here's something that might help.

In this recent study, researchers looked at the downstream effects of a new condominium tower in Honolulu called The Central Ala Moana. Completed in 2021, the building contains 512 units, of which 60% are income-restricted (310 units) and 40% are market-rate (202 units). It was developed under a state affordable-housing program that gave the developer height and density bonuses, plus fee waivers totalling about $13 million in exchange for delivering income-restricted units. (In my opinion, this is directionally preferable to unfunded inclusionary zoning mandates.)

Using address-history microdata, the researchers tracked who moved into the new condominium tower, and constructed detailed vacancy chains across multiple rounds of moves. Here's what they discovered:

Among documented vacancies, the 202 market-rate units produced 87 downstream vacancies (0.43 vacancies per initial unit), while the 310 income-restricted units produced 90 (0.29 vacancies per unit). Thus, market-rate units are more likely to generate a downstream vacancy. The main mechanism is new household formation: movers into income-restricted units are more likely to be a newly formed household, leaving family or roommates at the prior address and thus preventing a vacancy from being created.

In absolute numbers, they found that the completion of the building induced more than 500 local vacancies in the three years after construction, by setting off a chain of moves. Importantly, the researchers also found that the homes being vacated were, on average, about 40% less expensive than those in The Central. So even though a new building may be more expensive than the existing housing stock (which is generally the case or else the development wouldn't happen), it does generate benefits.