I appreciate the clarification on codes versus zoning.
However I did some digging and found that there's a big difference between what the commonly used international residential codes allow, and what zoning commonly allows.
From an article on tiny houses [0]:
> Most of the country’s local building codes have been adopted from the International Residential Code (IRC) for one- and two-family dwellings, which contains size specifications like rooms (except bathrooms and kitchens) must be at least 70 square feet, while ceiling height must be at least 7 feet.
> Zoning regulations are based off more local factors, and determine the size requirements of your home based on what zone it’s located in. You will need to call your local zoning or planning department to find that info. Many cities and counties, however, have a minimum size requirement of 1,000 square feet or more for construction of a new home on its own land, according to Tiny House Community.
And from a separate linked article on minimum dwelling size to satisfy codes [1]:
> I find no requirement that the sleeping area or kitchen must be in separate rooms, and instead could be combined in a studio arrangement. So I believe it could be legitimately argued that a minimum legal area by 2015 IRC standards could be as small as one habitable room of 70 sq. ft. with sleeping area and kitchen, plus a bathroom as small as 18 sq. ft. while meeting minimum spacing requirements.
So it does kind of look like zoning is still the big cost multiplier, since codes say 88 square feet minimum (separate bathroom), while zoning in most cities/counties in the US mandate 1000 square feet minimum. Quite a difference. And that's before getting into minimum lot size.
[0] https://archive.curbed.com/2016/9/22/13002832/tiny-house-zon...
[1] https://americantinyhouseassociation.org/minimum-size-for-a-...