Tend to disagree a bit here. Did a job recently where I ran to a gutter and got the termination above the down pipe on a combined (cast) system. Alternative was a Soakaway, I though it was a better option so ran in 40mm. the house was an awkward shape so I was able to run out the eaves along a wall about two feet and a swept elbow to above the down pipe. Nothing says you can't but I would rather run 10 meters inside than even a couple of meters outside if possible.
Seen a few shockers in my time including on several occasions condensate pipe being treated like overflows, sticking out walls going nowhere and pumped out onto roofs.