The office has always been on the second floor, ever since the last rebuild (IE, after the fridge-monster-in-the-basement storyline.)
The but with the kids sledding in through the window and using the V8 snowblower were gags- IE, the snow was deep enough to reach the second story. (Which isn't much of an exaggeration- I've personally seen 8 feet of snow accumulate in a winter, and there's places, like Valdez and Girdwood, where they've gotten 10, 12 and even 16 feet. The record in Valdez is something like just short of 600 inches (!!) in one season.)
Note that one gag in that winter storyline had Sandy walking upstairs to Doc's office, only to see Swampy walking by the window with the snowblower.
I might not have details like exactly where the breakroom is, or the precise layout of the upstairs area, but things like some rooms being upstairs, some being downstairs, and others being on the main floor, are actually pretty consistent.