This an older thread, and comments start with this being acceptable, and then shift to this being unacceptable but with no clear cut information prohibiting the practice.
Installation instruction for all products come from the manufacturers. The code requires installers to follow those instructions.
GAF has a technical bulletin they sent out years ago telling installers why it is a bad idea to dump downspout onto lower shingle roofs. Google GAF Technical Advisory Bulletin No: TABR20 1 1150.
It clears explains why damage to shingles associated with gutter downspout discharge is not covered by the manufacturer’s warranty.
I call it out everywhere I see it, and i see it everywhere in my area. On new homes and new gutter installs, I send out that GAF bulletin (even when not GAF shingles), and, almost always, the gutter guy comes back to extend the downspout to a lower gutter or to ground.
I have checked with other shingle manufacturers to see if they have anything in writing, and while they all agree with GAF, most dont have anything in writing… Except, they all seem to agree that their warranty does not cover damage to the shingles associated with downspout discharge.
The gutter guy is not the shingle guy oftentimes, and the gutter guy is causing damage to the shingles, albeit slow, long term damage they may not be noticeable for 10 years or more. The gutter guy needs to be aware of the damage he is causing. Ask him for a 20 year warranty on the shingles where he discharged his gutter… he likely will realize it is cheaper to install a short downspout extension to a lower gutter.
Fyi … looks better to use black downspout material on the roof surface, than white.
Secure the extension at the ends, not into the shingles for obvious reasons. If extension is over 10ft , think about other options… i. e., move downspout to different location.
Have fun out there!