So without the aid of the internet I found out where the building of Dunder Mifflin is.

I've always noticed that there are trains outside, and being the train nerd I am, i recognized the rail yard in the episode where they launch the watermelon from the rooftop onto the trampoline

its near the west end just before the rotunda of Saticoy and Woodman in Van Nuys, CA

I'm gonna take pics one of these days haha