I guess if you lived in a country where there weren't legal limits on subdivision size that would make sense. Is that the case in the USA?

"I live in a trailer on my parents backyard" doesn't quite have the same ring to it as "I built my own house at 19 you lazy bastard".

Generally, in the US, tiny homes are below legal minimum sizes for permanent homes. So, most are built as trailers, even if installed permanently.

Similarly, most towns and counties are subdivided such that tiny plots aren’t available.

There is a growing movement to allow ADUs in urban spaces, but it’s far from universal. If memory serves, Arlington VA (next to DC) allows an ADU within the main house (converted basement) with some fairly strict limits. No ADU in the back yard.

Basement apartments are problematic from a fire code perspective, especially if it's a "true" ADU that doesn't connect into the main house.

Correct. The basement ADUs I've seen in this area are on partially exposed "walk out" basements, so there's a dedicated door and window(s) for egress. Most retain a door to the main area, just dead-bolted (so no use in a fire).

That would be a problem. There seems to be a lack of tiny plots to go with tiny houses. I get the feeling cities are about as happy with any concept of tiny plots as they are of trailer parks.

I would imagine if you allowed tiny subdivisions you'd end up with slums, not hipsters instagramming their tiny houses.

This. Tiny house plots aren't distinctly different than trailer parks, and tiny houses aren't that distinct from a single wide.

There are nice trailer parks with new or maintained and addons porches. But there are also trailer parks where maintenance, but particularly septic and sewage were ignored

And that's why we won't get affordable housing.