The apartment (flats) issue is one that has some exceptions, in the USA, as I've seen single family homes and small duplexes, along with larger complexes that have inaccessible units - I think over a certain size, at least some percentage have to be accessible (just as a bathroom has to have at least one accessible stall, but is not required to have more).

Residences and rental units are not required to be ADA compliant. A multi-tenant residential building must be ADA compliant in the common and public areas of the building.

For residential rental units, landlords must make ‘reasonable accommodations, unless they would impose undue financial or administrative burden,’ which means if you get paralyzed while renting an apartment that isn’t ADA compliant you’re probably fucked unless you can afford to add a ramp yourself, and pay to have it removed when you leave. Adding a wheelchair ramp would be an undue financial burden, so a landlord isn’t required to add one. Replacing the door to a unit with a 36” wide door would likely not be considered an undue financial burden, so if you live in a building with ADA compliant public spaces and elevators, you’re probably OK since the landlord will likely install a larger door.

Government owned housing is required to be ADA compliant.