Apartment living defines Paris. ~80% of Parisians live in apartments (vs. ~15% in London, ~5% in NYC). Four forces cemented this:
By the Middle Ages, Paris was dense and walled. Vertical building on narrow medieval lots created the first apartment-style dwellings—multi-story, shared stairs, no elevators.
Baron Haussmann, under Napoleon III, demolished medieval warrens and built uniform limestone boulevards with 5–7 story buildings: ground-floor shops, mezzanine, noble étage (high ceilings, balconies), upper floors, mansard attics. This template became Paris's architectural DNA.
Paris is 105 km² for 2.1M people (20,000/km²). Land is exhausted. Detached houses are luxury rarities (Neuilly, Auteuil, 16th arr.). Apartments maximize housing per hectare.
Suburbs (banlieues) have more houses, but intra-muros Paris is and will remain an apartment city.