In residential application there is no requirement that I know of.
There is a huge part of the commercial application in commercial plumbing licencing (so the building won’t collapse). Try looking there.
I rant about gutter installation all the time.
Thanks for the info John, however the downspout capacity is not the root of the problem in many cases.
1200 sf roof needs one downspout, but 2 feet of gutter can not handle 1200 sf. Also every 90 degree fitting slows water flow and results in over/back flow. This is what I see all the time.
When there is water in the basement/crawlspace, I always point attention towards the roof (then downspout diversion and site grading).
Gutters are the primary cause of structural damage around here due to expansive soils.
This picture is a “mild” case from yesterdays inspection (it gets much worse).