Kilroy announced Dropbox's full-building lease at 333 Brannan yesterday, but we just learned from sources that the cloud storage giant is definitely signing a lease—which had been rumored—for the building next door. (It's going to slowly consume all of Brannan, just like The Blob.)

Dropbox will take all 100k SF of 345 Brannan, which means it'll lease 280k SF on the same SoMa block. (Just like its GB storage sizes, it appears to like lots of square footage.) Kilroy (its CEO John Kilroy Jr., above) is shooting for LEED Platinum at his William McDonough + Partners-designed 333, which will have a rooftop garden and slash water and energy usage. We heard that Pinterest was also seriously eyeing 333 (we broke news about its needing 200k SF this year). CBRE reps both tenants. Breevast's five-story building at 345, designed by Charles Bloszies, is expected to break ground soon; its neighbor at 333 got a head start in December.

Dropbox will also expand by 100k SF at its current home in China Basin's Wharfside Building (when UCSF vacates a floor in 2015, Dropbox will swoop in). Dropbox's grand total after that? Nearly half a million square feet in S.F. With 333 out, Pinterest is scrambling for its needed space and there's not a ton of new options, say sources. One is Boston Properties' 535 Mission, and sources say Pinterest would be a good fit there. Like Yelp, which picked 140 New Montgomery, the tech startup is used to sitting in smaller floor plates (its SoMa HQ, above).