"The Haussmann block mixed incomes vertically, by floor, so no single district rose or fell as one."
yes, and importantly, it did not achieve that diversity by requiring developers to sell 20 percent of units at prices affordable to households earning 60â80% of AMI.
Mixed income was partly a natural consequence of the building type itself. The same building contained larger and smaller apartments, premium and secondary floors, and units serving households with different incomes and needs. Crucially, it provided apartments attractive enough to retain upper- and middle-income households in the city, rather than pushing them to single family mansion neighborhoods.
Contemporary cities could recover some of that diversity by incentivizing a similar fine-grained apartment type: roughly 65 feet wide, 40 feet deep, and six stories, producing about 15,600 gross square feet and perhaps 14 apartments averaging around 1,000 square feet.
Rather than forcing a small building to absorb deeply discounted units, a city could incentivize ~20 percent of its apartments to accept housing vouchers.
Typology sets the grain of the mix. The Haussmann block mixed incomes vertically, by floor, so no single district rose or fell as one. US zoning sorts horizontally instead. Paris's smoother gradient is the building doing what the zoning code won't.