Could We Boost Housing Construction by Permitting ADUs Under the HUD Code?

Talks of $80 to $300 per square foot construction costs.

20 years ago, with extreme measures to save costs, salvaged materiels, and so on; I built a house for $25 per sq ft. Online calculator says that the equivalent of $42 / today. That's half buried 1.5 floors and 3k sq ft.

Perhaps we can cut the cost of housing by making houses cheaper? Do we really need layers of flooring when a cement slab and epoxy coat on top of that make a resilient, useful floor for decades?

How much of that expense is classed as "trim" that could be left out entirely without degrading any of the home's functionality?

I won't even start on the things like GFCI / "Arc fault" breakers, which would be great if they worked as advertised but are required even though they really don't.

We have a gap in home construction where the "tar paper shack" used to be; we've decided as a society that housing must be high standard or not there. This error causes many of the problems we're concerned with.