Now that Dropbox officially signed at 345 Brannan yesterday—tacking on 100k SF to its neighboring 180k SF lease inked Friday at 333 Brannan—we asked sources close to the deal for an insider's view. Turns out underground tunnels to connect the two buildings will be involved. (As is common with unnamed sources.)

While 345 (rendered above) is trailing 333 by a couple months due to site permits, it's smaller and will catch up to deliver at the same time for spring 2015 occupancy. The buildings will stand about a foot apart.) It's the first time one source has done a deal where a tenant wanted to move into two next-door buildings with different landlords that are "healthy competitors." (Is it too late for Nobel Peace Prize nominations?) When he first heard about Dropbox's plan four or five months ago, he admits it sounded completely crazy. Could Kilroy and Breevast really jump into bed together for their mutual benefit?

The fact is everyone wants to be in SoMa, he says, and they're no longer picky about having exposed brick and timber; there's none of that left for a 300k SF tenant. That's also where the young workforce wants to live and bike to work. He says Salesforce and Dropbox are S.F.'s two big tigers sucking up space, similar to Seattle's Amazon and Microsoft. Pinterest also wanted 333 (above) but didn't get to the plate quickly enough, he adds. It was not just about the landlord; it was about Dropbox being on its game, too.