Floor Area Ratio regulations are pretty standard rules.

However, rules like these are "zoning/planning" and are usually set by the city or county, not the state.

The county shifting to Idaho is likely to have no effect on that regulation.

FAR's are why Palo Alto had to bring in regulations on basement sizes (the majority of the house was underground!), and why Zuckerberg is having problems with his renovation plans.

https://en.wikipedia.org/wiki/Floor_area_ratio

https://www.mercurynews.com/2017/08/31/palo-alto-zuckerberg-...

https://padailypost.com/2019/08/26/is-city-going-to-far-with...

It's meant to affect light so should not apply to basements. Finished basement area is still taxed in most counties.

If I were living in that state, I would build a fractal roof for my dog house that had close to an infinite surface area and then I would sue myself to teach people how dumb that rule is.