Tej Parikh writing in the FT:

…high-end developments unlock long housing chains. As higher-income households move into newly built units, they free up older properties, raising supply and slashing prices for middle- and lower-end housing through a process known as filtering. Numerous international studies underscore this positive ripple effect.

One published last year tracked households that moved into a newly built 512-unit condominium tower in Honolulu, Hawaii. It found that the building created at least 557 vacancies in older and cheaper apartments across the city in just three years, with market-rate units more likely to release the largest chains.

Filtering can also be widespread. A 2021 study in Helsinki using geo-coded population data found that every 100 new market-rate units in the city centre led to around 60 units becoming available in the city’s bottom half of neighbourhoods by income. An analysis across all homes in Sweden over several decades concluded that “new homes, even those initially primarily inhabited by rich people, lead to substantial trickle-down effects that also benefit the poor”.

Other US studies highlight how market-rate developments benefit less well-to-do local residents by lowering housing costs. In San Francisco, a 2021 paper found new developments lowered the risk of eviction notices for residents in rent-stabilised housing. Even “luxury” developments in New York City — which Mamdani has criticised — have been shown to contribute to lower local rents and sales prices.

or David Attenborough: