well first off there is no basement now. but i see what you want, some passage to sneak through the map.
and as for the backyard tower, i was thinking hard about this because right now.. it seems cts will get raped. the terrorists have the rooftop, extended and the sniper tower on top of all the areas in regular office at backyard. as for the cts they have 1 garage door and the stair, i can see a lot of ct deaths there right now.
i am thinking of making the sniper tower exclusively the cts to help them out. and a back way to extended sounds like a great idea to me, il just have to figure out what will work.

how about..
i open that garage door in ct spawn and have it lead outside. to a new outdoor area similar to backyard, cts will be able to secure it first and work their way into extended from there. i think that should work.