Would it be at all possible to use the source SDK (downloaded from Steam) to develop an entire independent game, not using any preexisting levels or classes specific to a game? Or am I forced to only use assets and classes from an existing game using it's own authoring tools? What would making a separate game take? I haven't used the source SDK yet, and from what I've been researching, it seems like it would be just a mod tool in its current state. Is this all it is? Or is it actually a software development kit?

Also, sorry if I'm posting in the wrong forum, but I'm pretty sure the source SDK falls under 'Tools and APIs'.